diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..418f728
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,42 @@
+## Ruby
+*.gem
+*.rbc
+/.config
+/coverage/
+/InstalledFiles
+/pkg/
+/spec/reports/
+/spec/tmp/
+/spec/examples.txt
+/test/tmp/
+/test/version_tmp/
+/tmp/
+
+## Used by dotenv library to load environment variables
+.env
+
+## Documentation cache and generated files:
+/.yardoc/
+/_yardoc/
+/doc/
+/rdoc/
+/coverage/
+
+## Environment normalization:
+/.bundle/
+/vendor/bundle
+/lib/bundler/man/
+
+## OS generated files
+.DS_Store
+.DS_Store?
+._*
+.Spotlight-V100
+.Trashes
+ehthumbs.db
+Thumbs.db
+
+## Other
+/.bundle/
+Gemfile.lock
+.rvmrc
\ No newline at end of file
diff --git a/.travis.yml b/.travis.yml
new file mode 100644
index 0000000..2f34bc3
--- /dev/null
+++ b/.travis.yml
@@ -0,0 +1,4 @@
+language: ruby
+before_install: gem update --system
+script: bundle exec rake
+rvm: 2.5
\ No newline at end of file
diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..069de28
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,3 @@
+# 0.1.0
+
+Initial commit! 🎉
\ No newline at end of file
diff --git a/Gemfile b/Gemfile
new file mode 100644
index 0000000..851fabc
--- /dev/null
+++ b/Gemfile
@@ -0,0 +1,2 @@
+source 'https://rubygems.org'
+gemspec
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..28041b1
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2019 Edwin Onuonga
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
\ No newline at end of file
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..6b6c4e3
--- /dev/null
+++ b/README.md
@@ -0,0 +1,3 @@
+# Arx
+
+A Ruby interface for querying academic papers on the arXiv search API.
\ No newline at end of file
diff --git a/Rakefile b/Rakefile
new file mode 100644
index 0000000..10119b7
--- /dev/null
+++ b/Rakefile
@@ -0,0 +1,7 @@
+require 'bundler/gem_tasks'
+require 'rspec/core/rake_task'
+
+desc 'Run application specs'
+RSpec::Core::RakeTask.new :spec
+
+task default: [:spec]
\ No newline at end of file
diff --git a/arx.gemspec b/arx.gemspec
new file mode 100644
index 0000000..d61a8b6
--- /dev/null
+++ b/arx.gemspec
@@ -0,0 +1,24 @@
+lib = File.expand_path('../lib', __FILE__)
+$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
+require 'arx/version'
+
+Gem::Specification.new do |spec|
+  spec.name          = 'arx'
+  spec.version       = Arx::VERSION
+  spec.authors       = ['Edwin Onuonga']
+  spec.email         = ['edwinonuonga@gmail.com']
+
+  spec.summary       = %q{A Ruby interface for querying academic papers on the arXiv search API.}
+  spec.license       = 'MIT'
+  spec.require_paths = ['lib']
+  spec.files         = Dir.glob('lib/**/*', File::FNM_DOTMATCH) + %w[
+    Gemfile LICENSE CHANGELOG.md README.md Rakefile arx.gemspec
+  ]
+
+  spec.add_runtime_dependency 'nokogiri', '~> 1.10'
+  spec.add_runtime_dependency 'nokogiri-happymapper', '~> 0.8'
+
+  spec.add_development_dependency 'bundler', '~> 2.0'
+  spec.add_development_dependency 'rake', '~> 12.3'
+  spec.add_development_dependency 'rspec', '~> 3.7'
+end
\ No newline at end of file
diff --git a/lib/arx.rb b/lib/arx.rb
new file mode 100644
index 0000000..a0481de
--- /dev/null
+++ b/lib/arx.rb
@@ -0,0 +1,53 @@
+# frozen_string_literal: true
+
+require 'nokogiri'
+require 'open-uri'
+require 'arx/version'
+require 'arx/categories'
+require 'arx/query/query'
+require 'arx/query/validate'
+require 'arx/entities/author'
+require 'arx/entities/category'
+require 'arx/entities/paper'
+
+# A Ruby interface for querying academic papers on the arXiv search API.
+module Arx
+
+  # The arXiv search API endpoint.
+  ENDPOINT = 'http://export.arxiv.org/api/query?'
+
+  # Performs a search query for papers on the arXiv search API.
+  #
+  # @param ids [Array<String>] The IDs of the arXiv papers to restrict the query to.
+  # @param sort_by [Symbol] The sorting criteria for the returned results (see {Query::SORT_BY}).
+  # @param sort_order [Symbol] The sorting order for the returned results (see {Query::SORT_ORDER}).
+  # @return [Array<Paper>, Paper] The {Paper}(s) found by the search query.
+  def self.search(*ids, sort_by: :relevance, sort_order: :descending)
+    query = Query.new(*ids, sort_by: sort_by, sort_order: sort_order)
+
+    yield query if block_given?
+
+    document = Nokogiri::XML open(ENDPOINT + query.to_s + '&max_results=10000')
+    document.remove_namespaces!
+
+    results = Paper.parse(document, single: false).reject {|paper| paper.id.empty?}
+    raise MissingPaper.new(ids.first) if results.empty? && ids.size == 1
+    ids.size == 1 && results.size == 1 ? results.first : results
+  end
+end
+
+# Performs a search query for papers on the arXiv search API.
+#
+# @note This is an alias of the {Arx.search} method.
+# @see Arx.search
+# @param ids [Array<String>] The IDs of the arXiv papers to restrict the query to.
+# @param sort_by [Symbol] The sorting criteria for the returned results (see {Arx::Query::SORT_BY}).
+# @param sort_order [Symbol] The sorting order for the returned results (see {Arx::Query::SORT_ORDER}).
+# @return [Array<Paper>, Paper] The {Arx::Paper}(s) found by the search query.
+def Arx(*ids, sort_by: :relevance, sort_order: :descending, &block)
+  if block_given?
+    Arx.search *ids, sort_by: sort_by, sort_order: sort_order, &block
+  else
+    Arx.search *ids, sort_by: sort_by, sort_order: sort_order
+  end
+end
\ No newline at end of file
diff --git a/lib/arx/categories.rb b/lib/arx/categories.rb
new file mode 100644
index 0000000..88bae94
--- /dev/null
+++ b/lib/arx/categories.rb
@@ -0,0 +1,161 @@
+# frozen_string_literal: true
+
+module Arx
+
+  # arXiv categories and their full names
+  CATEGORIES = {
+    'astro-ph' => 'Astrophysics',
+    'astro-ph.CO' => 'Cosmology and Nongalactic Astrophysics',
+    'astro-ph.EP' => 'Earth and Planetary Astrophysics',
+    'astro-ph.GA' => 'Astrophysics of Galaxies',
+    'astro-ph.HE' => 'High Energy Astrophysical Phenomena',
+    'astro-ph.IM' => 'Instrumentation and Methods for Astrophysics',
+    'astro-ph.SR' => 'Solar and Stellar Astrophysics',
+    'cond-mat.dis-nn' => 'Disordered Systems and Neural Networks',
+    'cond-mat.mes-hall' => 'Mesoscale and Nanoscale Physics',
+    'cond-mat.mtrl-sci' => 'Materials Science',
+    'cond-mat.other' => 'Other Condensed Matter',
+    'cond-mat.quant-gas' => 'Quantum Gases',
+    'cond-mat.soft' => 'Soft Condensed Matter',
+    'cond-mat.stat-mech' => 'Statistical Mechanics',
+    'cond-mat.str-el' => 'Strongly Correlated Electrons',
+    'cond-mat.supr-con' => 'Superconductivity',
+    'cs.AI' => 'Artificial Intelligence',
+    'cs.AR' => 'Hardware Architecture',
+    'cs.CC' => 'Computational Complexity',
+    'cs.CE' => 'Computational Engineering, Finance, and Science',
+    'cs.CG' => 'Computational Geometry',
+    'cs.CL' => 'Computation and Language',
+    'cs.CR' => 'Cryptography and Security',
+    'cs.CV' => 'Computer Vision and Pattern Recognition',
+    'cs.CY' => 'Computers and Society',
+    'cs.DB' => 'Databases',
+    'cs.DC' => 'Distributed, Parallel, and Cluster Computing',
+    'cs.DL' => 'Digital Libraries',
+    'cs.DM' => 'Discrete Mathematics',
+    'cs.DS' => 'Data Structures and Algorithms',
+    'cs.ET' => 'Emerging Technologies',
+    'cs.FL' => 'Formal Languages and Automata Theory',
+    'cs.GL' => 'General Literature',
+    'cs.GR' => 'Graphics',
+    'cs.GT' => 'Computer Science and Game Theory',
+    'cs.HC' => 'Human-Computer Interaction',
+    'cs.IR' => 'Information Retrieval',
+    'cs.IT' => 'Information Theory',
+    'cs.LG' => 'Learning',
+    'cs.LO' => 'Logic in Computer Science',
+    'cs.MA' => 'Multiagent Systems',
+    'cs.MM' => 'Multimedia',
+    'cs.MS' => 'Mathematical Software',
+    'cs.NA' => 'Numerical Analysis',
+    'cs.NE' => 'Neural and Evolutionary Computing',
+    'cs.NI' => 'Networking and Internet Architecture',
+    'cs.OH' => 'Other Computer Science',
+    'cs.OS' => 'Operating Systems',
+    'cs.PF' => 'Performance',
+    'cs.PL' => 'Programming Languages',
+    'cs.RO' => 'Robotics',
+    'cs.SC' => 'Symbolic Computation',
+    'cs.SD' => 'Sound',
+    'cs.SE' => 'Software Engineering',
+    'cs.SI' => 'Social and Information Networks',
+    'cs.SY' => 'Systems and Control',
+    'econ.EM' => 'Econometrics',
+    'eess.AS' => 'Audio and Speech Processing',
+    'eess.IV' => 'Image and Video Processing',
+    'eess.SP' => 'Signal Processing',
+    'gr-qc' => 'General Relativity and Quantum Cosmology',
+    'hep-ex' => 'High Energy Physics - Experiment',
+    'hep-lat' => 'High Energy Physics - Lattice',
+    'hep-ph' => 'High Energy Physics - Phenomenology',
+    'hep-th' => 'High Energy Physics - Theory',
+    'math.AC' => 'Commutative Algebra',
+    'math.AG' => 'Algebraic Geometry',
+    'math.AP' => 'Analysis of PDEs',
+    'math.AT' => 'Algebraic Topology',
+    'math.CA' => 'Classical Analysis and ODEs',
+    'math.CO' => 'Combinatorics',
+    'math.CT' => 'Category Theory',
+    'math.CV' => 'Complex Variables',
+    'math.DG' => 'Differential Geometry',
+    'math.DS' => 'Dynamical Systems',
+    'math.FA' => 'Functional Analysis',
+    'math.GM' => 'General Mathematics',
+    'math.GN' => 'General Topology',
+    'math.GR' => 'Group Theory',
+    'math.GT' => 'Geometric Topology',
+    'math.HO' => 'History and Overview',
+    'math.IT' => 'Information Theory',
+    'math.KT' => 'K-Theory and Homology',
+    'math.LO' => 'Logic',
+    'math.MG' => 'Metric Geometry',
+    'math.MP' => 'Mathematical Physics',
+    'math.NA' => 'Numerical Analysis',
+    'math.NT' => 'Number Theory',
+    'math.OA' => 'Operator Algebras',
+    'math.OC' => 'Optimization and Control',
+    'math.PR' => 'Probability',
+    'math.QA' => 'Quantum Algebra',
+    'math.RA' => 'Rings and Algebras',
+    'math.RT' => 'Representation Theory',
+    'math.SG' => 'Symplectic Geometry',
+    'math.SP' => 'Spectral Theory',
+    'math.ST' => 'Statistics Theory',
+    'math-ph' => 'Mathematical Physics',
+    'nlin.AO' => 'Adaptation and Self-Organizing Systems',
+    'nlin.CD' => 'Chaotic Dynamics',
+    'nlin.CG' => 'Cellular Automata and Lattice Gases',
+    'nlin.PS' => 'Pattern Formation and Solitons',
+    'nlin.SI' => 'Exactly Solvable and Integrable Systems',
+    'nucl-ex' => 'Nuclear Experiment',
+    'nucl-th' => 'Nuclear Theory',
+    'physics.acc-ph' => 'Accelerator Physics',
+    'physics.ao-ph' => 'Atmospheric and Oceanic Physics',
+    'physics.app-ph' => 'Applied Physics',
+    'physics.atm-clus' => 'Atomic and Molecular Clusters',
+    'physics.atom-ph' => 'Atomic Physics',
+    'physics.bio-ph' => 'Biological Physics',
+    'physics.chem-ph' => 'Chemical Physics',
+    'physics.class-ph' => 'Classical Physics',
+    'physics.comp-ph' => 'Computational Physics',
+    'physics.data-an' => 'Data Analysis, Statistics and Probability',
+    'physics.ed-ph' => 'Physics Education',
+    'physics.flu-dyn' => 'Fluid Dynamics',
+    'physics.gen-ph' => 'General Physics',
+    'physics.geo-ph' => 'Geophysics',
+    'physics.hist-ph' => 'History and Philosophy of Physics',
+    'physics.ins-det' => 'Instrumentation and Detectors',
+    'physics.med-ph' => 'Medical Physics',
+    'physics.optics' => 'Optics',
+    'physics.plasm-ph' => 'Plasma Physics',
+    'physics.pop-ph' => 'Popular Physics',
+    'physics.soc-ph' => 'Physics and Society',
+    'physics.space-ph' => 'Space Physics',
+    'q-bio.BM' => 'Biomolecules',
+    'q-bio.CB' => 'Cell Behavior',
+    'q-bio.GN' => 'Genomics',
+    'q-bio.MN' => 'Molecular Networks',
+    'q-bio.NC' => 'Neurons and Cognition',
+    'q-bio.OT' => 'Other Quantitative Biology',
+    'q-bio.PE' => 'Populations and Evolution',
+    'q-bio.QM' => 'Quantitative Methods',
+    'q-bio.SC' => 'Subcellular Processes',
+    'q-bio.TO' => 'Tissues and Organs',
+    'q-fin.CP' => 'Computational Finance',
+    'q-fin.EC' => 'Economics',
+    'q-fin.GN' => 'General Finance',
+    'q-fin.MF' => 'Mathematical Finance',
+    'q-fin.PM' => 'Portfolio Management',
+    'q-fin.PR' => 'Pricing of Securities',
+    'q-fin.RM' => 'Risk Management',
+    'q-fin.ST' => 'Statistical Finance',
+    'q-fin.TR' => 'Trading and Market Microstructure',
+    'quant-ph' => 'Quantum Physics',
+    'stat.AP' => 'Applications',
+    'stat.CO' => 'Computation',
+    'stat.ME' => 'Methodology',
+    'stat.ML' => 'Machine Learning',
+    'stat.OT' => 'Other Statistics',
+    'stat.TH' => 'Statistics Theory'
+  }.freeze
+end
\ No newline at end of file
diff --git a/lib/arx/cleaner.rb b/lib/arx/cleaner.rb
new file mode 100644
index 0000000..c8d54db
--- /dev/null
+++ b/lib/arx/cleaner.rb
@@ -0,0 +1,13 @@
+module Arx
+
+  # Class for cleaning strings.
+  class Cleaner
+
+    # Cleans strings.
+    # @param [String] string Removes newline/return characters and multiple spaces from a string.
+    # @return [String] The cleaned string.
+    def self.clean(string)
+      string.gsub(/\r\n|\r|\n/, ' ').strip.squeeze ' '
+    end
+  end
+end
\ No newline at end of file
diff --git a/lib/arx/entities/author.rb b/lib/arx/entities/author.rb
new file mode 100644
index 0000000..fe36ae0
--- /dev/null
+++ b/lib/arx/entities/author.rb
@@ -0,0 +1,29 @@
+require 'happymapper'
+require 'arx/cleaner'
+
+module Arx
+
+  # Entity/model representing an arXiv paper's author.
+  class Author
+    include HappyMapper
+
+    tag 'author'
+
+    # @!method name
+    # The name of the author.
+    # @return [String]
+    element :name, Cleaner, tag: 'name', parser: :clean
+
+    # @!method affiliations
+    # The author's affiliations.
+    # @return [Array<String>]
+    has_many :affiliations, Cleaner, tag: 'affiliation', parser: :clean
+
+    # @!method affiliations?
+    # Whether or not the author has any affiliations.
+    # @return [Boolean]
+    def affiliations?
+      !affiliations.empty?
+    end
+  end
+end
\ No newline at end of file
diff --git a/lib/arx/entities/category.rb b/lib/arx/entities/category.rb
new file mode 100644
index 0000000..37231f6
--- /dev/null
+++ b/lib/arx/entities/category.rb
@@ -0,0 +1,24 @@
+require 'arx/categories'
+require 'arx/cleaner'
+
+module Arx
+
+  # Entity/model representing an arXiv paper's category.
+  class Category
+    include HappyMapper
+
+    tag 'category'
+
+    # @!method name
+    # The abbreviated name of the category.
+    # @return [String]
+    attribute :name, Cleaner, parser: :clean, tag: 'term'
+
+    # The full name of the category.
+    # @see CATEGORIES
+    # @return [String]
+    def full_name
+      CATEGORIES[term]
+    end
+  end
+end
\ No newline at end of file
diff --git a/lib/arx/entities/link.rb b/lib/arx/entities/link.rb
new file mode 100644
index 0000000..90e820f
--- /dev/null
+++ b/lib/arx/entities/link.rb
@@ -0,0 +1,22 @@
+require 'happymapper'
+
+module Arx
+
+  # Helper entity/model representing a link on an arXiv paper.
+  class Link
+    include HappyMapper
+
+    tag 'link'
+
+    attribute :title, String
+    attribute :rel,   String
+    attribute :type,  String
+    attribute :href,  String
+
+    %w[pdf doi].each do |link_type|
+      define_method "#{link_type}?" do
+        @title == link_type
+      end
+    end
+  end
+end
\ No newline at end of file
diff --git a/lib/arx/entities/paper.rb b/lib/arx/entities/paper.rb
new file mode 100644
index 0000000..a31a5f8
--- /dev/null
+++ b/lib/arx/entities/paper.rb
@@ -0,0 +1,159 @@
+require 'happymapper'
+require 'arx/exceptions'
+require 'arx/cleaner'
+require_relative 'author'
+require_relative 'category'
+require_relative 'link'
+
+module Arx
+
+  # Entity/model representing an arXiv paper.
+  class Paper
+    include HappyMapper
+
+    tag 'entry'
+
+    element :id, Cleaner, parser: :clean, tag: 'id'
+    # The identifier of the paper.
+    # @note This is either in {Validate::OLD_IDENTIFIER_FORMAT} or {Validate::NEW_IDENTIFIER_FORMAT}.
+    # @example
+    #   1705.01662v1
+    #   cond-mat/0211034
+    # @return [String] The paper's identifier.
+    def id
+      @id.sub /https?\:\/\/arxiv\.org\/abs\//, ''
+    end
+
+    # The URL of the paper on the arXiv website.
+    # @example
+    #   http://arxiv.org/abs/1705.01662v1
+    #   http://arxiv.org/abs/cond-mat/0211034
+    # @return [String] The paper's arXiv URL.
+    def url
+      @id
+    end
+
+    # @!method last_updated
+    # The date that the paper was last updated.
+    # @return [DateTime]
+    element :last_updated, DateTime, tag: 'updated'
+
+    # @!method publish_date
+    # The original publish/submission date of the paper.
+    # @return [DateTime]
+    element :publish_date, DateTime, tag: 'published'
+
+    # @!method title
+    # The title of the paper.
+    # @return [DateTime]
+    element :title, Cleaner, parser: :clean, tag: 'title'
+
+    # @!method authors
+    # The authors of the paper.
+    # @return [Array<Author>]
+    has_many :authors, Author, tag: 'author'
+
+    # @!method primary_category
+    # The primary category of the paper.
+    # @return [Category]
+    element :primary_category, Category, tag: 'primary_category'
+
+    # @!method categories
+    # The categories of the paper.
+    # @return [Array<Category>]
+    has_many :categories, Category, tag: 'category'
+
+    # Whether the paper is a revision or not.
+    # @note A paper is a revision if {last_updated} differs from {publish_date}.
+    # @return [Boolean]
+    def revision?
+      @publish_date != @last_updated
+    end
+
+    # @!method summary
+    # The summary (or abstract) of the paper.
+    # @return [String]
+    element :summary, Cleaner, parser: :clean, tag: 'summary'
+    alias_method :abstract, :summary
+
+    # @!method comment?
+    # Whether or not the paper has a comment.
+    # @return [Boolean]
+
+    # @!method comment
+    # The comment of the paper.
+    # @note This is an optional metadata field on an arXiv paper. To check whether the paper has a comment, use {comment?}
+    # @raise {MissingFieldError} If the paper does not have a comment.
+    # @return [String]
+    element :comment, Cleaner, parser: :clean, tag: 'comment'
+
+    # @!method journal?
+    # Whether or not the paper has a journal reference.
+    # @return [Boolean]
+
+    # @!method journal
+    # The journal reference of the paper.
+    # @note This is an optional metadata field on an arXiv paper. To check whether the paper has a journal reference, use {journal?}
+    # @raise {MissingFieldError} If the paper does not have a journal reference.
+    # @return [String]
+    element :journal, Cleaner, parser: :clean, tag: 'journal_ref'
+
+    %i[comment journal].each do |optional|
+      exists = "#{optional}?"
+
+      define_method exists do
+        !instance_variable_get("@#{optional}").empty?
+      end
+
+      define_method optional do
+        if self.send "#{optional}?"
+          instance_variable_get("@#{optional}")
+        else
+          raise MissingFieldError.new(optional)
+        end
+      end
+    end
+
+    has_many :links, Link, tag: 'link'
+
+    # @!method pdf?
+    # Whether or not the paper has a PDF link.
+    # @return [Boolean]
+
+    # @!method pdf_url
+    # Link to the PDF version of the paper.
+    # @note This is an optional metadata field on an arXiv paper. To check whether the paper has a PDF link, use {pdf?}
+    # @raise {MissingLinkError} If the paper does not have a PDF link.
+    # @return [String]
+
+    # @!method doi?
+    # Whether or not the paper has a DOI (Digital Object Identifier) link.
+    # @see https://arxiv.org/help/jref#doi
+    # @see https://arxiv.org/help/prep#doi
+    # @return [Boolean]
+
+    # @!method doi_url
+    # Link to the DOI (Digital Object Identifier) of the paper.
+    # @see https://arxiv.org/help/jref#doi
+    # @see https://arxiv.org/help/prep#doi
+    # @note This is an optional metadata field on an arXiv paper. To check whether the paper has a DOI link, use {doi?}
+    # @raise {MissingLinkError} If the paper does not have a DOI link.
+    # @return [String]
+
+    %i[pdf doi].each do |link_type|
+      exists = "#{link_type}?".to_sym
+
+      define_method exists do
+        links.any? &exists
+      end
+
+      define_method "#{link_type}_url" do
+        if self.send exists
+          links.find(&exists).href
+        else
+          raise MissingLinkError.new link_type.to_s.upcase
+        end
+      end
+    end
+  end
+end
\ No newline at end of file
diff --git a/lib/arx/exceptions.rb b/lib/arx/exceptions.rb
new file mode 100644
index 0000000..21663ba
--- /dev/null
+++ b/lib/arx/exceptions.rb
@@ -0,0 +1,23 @@
+module Arx
+
+  # Custom error for missing links on an arXiv paper.
+  class MissingLinkError < StandardError
+    def initialize(link_type)
+      super "This arXiv paper does not have a #{link_type} link"
+    end
+  end
+
+  # Custom error for missing fields on an arXiv paper.
+  class MissingFieldError < StandardError
+    def initialize(field)
+      super "This arXiv paper is missing the `#{field}` field"
+    end
+  end
+
+  # Custom error for missing arXiv papers.
+  class MissingPaper < StandardError
+    def initialize(id)
+      super "Couldn't find an arXiv paper with ID: #{id}"
+    end
+  end
+end
\ No newline at end of file
diff --git a/lib/arx/query/query.rb b/lib/arx/query/query.rb
new file mode 100644
index 0000000..21b13c4
--- /dev/null
+++ b/lib/arx/query/query.rb
@@ -0,0 +1,266 @@
+# frozen_string_literal: true
+
+require 'cgi'
+require_relative 'validate'
+
+module Arx
+
+  # Class for generating arXiv search API query strings.
+  #
+  # @attr query [String] The string representing the search query.
+  class Query
+
+    # Mapping for URL query parameters supported by the arXiv search API.
+    PARAMS = {
+      search_query: 'search_query',
+      id_list: 'id_list',
+      sort_by: 'sortBy',
+      sort_order: 'sortOrder'
+    }
+
+    # Logical connectives supported by the arXiv search API.
+    CONNECTIVES = {
+      and: 'AND',
+      or: 'OR',
+      and_not: 'ANDNOT'
+    }
+
+    # Logical connective method names.
+    CONNECTIVE_METHODS = {
+      '&': :and,
+      '!': :and_not,
+      '|': :or
+    }
+
+    # Supported fields for the search queries made to the arXiv search API.
+    # @see https://arxiv.org/help/prep arXiv metadata fields
+    # @see https://arxiv.org/help/api/user-manual#query_details arXiv user manual (query details)
+    FIELDS = {
+      title: 'ti',     # Title
+      author: 'au',    # Author
+      abstract: 'abs', # Abstract
+      comment: 'co',   # Comment
+      journal: 'jr',   # Journal reference
+      category: 'cat', # Subject category
+      report: 'rn',    # Report number
+      all: 'all'       # All (of the above)
+    }
+
+    # Supported criteria for the +sortBy+ parameter.
+    SORT_BY = {
+      relevance: 'relevance',
+      last_updated: 'lastUpdated',
+      date_submitted: 'submittedDate'
+    }
+
+    # Supported criteria for the +sortOrder+ parameter.
+    SORT_ORDER = {
+      ascending: 'ascending',
+      descending: 'descending'
+    }
+
+    # Initializes a new Query object.
+    #
+    # @param ids [Array<String>] The IDs of the arXiv papers to restrict the query to.
+    # @param sort_by [Symbol] The sorting criteria for the returned results (see {SORT_BY}).
+    # @param sort_order [Symbol] The sorting order for the returned results (see {SORT_ORDER}).
+    # @return [Query] The initialized query object.
+    def initialize(*ids, sort_by: :relevance, sort_order: :descending)
+      @query = String.new
+
+      Validate.sort_by sort_by, permitted: SORT_BY.keys
+      @query << "&#{PARAMS[:sort_by]}=#{SORT_BY[sort_by]}"
+
+      Validate.sort_order sort_order, permitted: SORT_ORDER.keys
+      @query << "&#{PARAMS[:sort_order]}=#{SORT_ORDER[sort_order]}"
+
+      ids.flatten!
+      unless ids.empty?
+        ids.map! {|id| extract_id id}
+        Validate.ids ids
+        @query << "&#{PARAMS[:id_list]}=#{ids * ','}"
+      end
+
+      yield self if block_given?
+    end
+
+    # @!method &
+    # Logical conjunction (+AND+) of subqueries.
+    # @see https://arxiv.org/help/api/user-manual#query_details arXiv user manual
+    # @return [self]
+
+    # @!method !
+    # Logical negated conjunction (+ANDNOT+) of subqueries.
+    # @see https://arxiv.org/help/api/user-manual#query_details arXiv user manual
+    # @return [self]
+
+    # @!method |
+    # Logical disjunction (+OR+) of subqueries.
+    # @see https://arxiv.org/help/api/user-manual#query_details arXiv user manual
+    # @return [self]
+
+    CONNECTIVE_METHODS.each do |symbol, connective|
+      define_method(symbol) { add_connective connective }
+    end
+
+    # @!method title(*values, exact: false, connective: :and)
+    # Search for papers by {https://arxiv.org/help/prep#title title}.
+    # @param values [Array<String>] Title(s) of papers to search for.
+    # @param exact [Boolean] Whether to search for an exact match of the title(s).
+    # @param connective [Symbol] The logical connective to use (see {CONNECTIVES}). Only applies if there are multiple values.
+    # @return [self]
+
+    # @!method author(*values, exact: false, connective: :and)
+    # Search for papers by {https://arxiv.org/help/prep#author author}.
+    # @param values [Array<String>] Author(s) of papers to search for.
+    # @param exact [Boolean] Whether to search for an exact match of the author's name(s).
+    # @param connective [Symbol] The logical connective to use (see {CONNECTIVES}). Only applies if there are multiple values.
+    # @return [self]
+
+    # @!method abstract(*values, exact: false, connective: :and)
+    # Search for papers by {https://arxiv.org/help/prep#abstract abstract}.
+    # @param values [Array<String>] Abstract(s) of papers to search for.
+    # @param exact [Boolean] Whether to search for an exact match of the abstract(s).
+    # @param connective [Symbol] The logical connective to use (see {CONNECTIVES}). Only applies if there are multiple values.
+    # @return [self]
+
+    # @!method comment(*values, exact: false, connective: :and)
+    # Search for papers by {https://arxiv.org/help/prep#comments comment}.
+    # @param values [Array<String>] Comment(s) of papers to search for.
+    # @param exact [Boolean] Whether to search for an exact match of the comment(s).
+    # @param connective [Symbol] The logical connective to use (see {CONNECTIVES}). Only applies if there are multiple values.
+    # @return [self]
+
+    # @!method journal(*values, exact: false, connective: :and)
+    # Search for papers by {https://arxiv.org/help/prep#journal journal reference}.
+    # @param values [Array<String>] Journal reference(s) of papers to search for.
+    # @param exact [Boolean] Whether to search for an exact match of the journal refernece(s).
+    # @param connective [Symbol] The logical connective to use (see {CONNECTIVES}). Only applies if there are multiple values.
+    # @return [self]
+
+    # @!method category(*values, connective: :and)
+    # Search for papers by {https://arxiv.org/help/prep#category category}.
+    # @param values [Array<String>] Category(s) of papers to search for.
+    # @param connective [Symbol] The logical connective to use (see {CONNECTIVES}). Only applies if there are multiple values.
+    # @return [self]
+
+    # @!method report(*values, connective: :and)
+    # Search for papers by {https://arxiv.org/help/prep#report report number}.
+    # @param values [Array<String>] Report number(s) of papers to search for.
+    # @param connective [Symbol] The logical connective to use (see {CONNECTIVES}). Only applies if there are multiple values.
+    # @return [self]
+
+    # @!method all(*values, exact: true, connective: :and)
+    # Search for papers by all fields (see {FIELDS}).
+    # @param values [Array<String>] Field value(s) of papers to search for.
+    # @param exact [Boolean] Whether to search for an exact match of the comment(s).
+    # @param connective [Symbol] The logical connective to use (see {CONNECTIVES}). Only applies if there are multiple values.
+    # @return [self]
+
+    FIELDS.each do |name, field|
+      define_method(name) do |*values, exact: true, connective: :and|
+        return if values.empty?
+
+        Validate.values values
+        Validate.categories values if name == :category
+        Validate.exact exact
+        Validate.connective connective, permitted: CONNECTIVES.keys
+
+        values.map! &CGI.method(:escape)
+
+        # Forms a field:value pair
+        pair = ->(value){"#{field}:#{exact ? enquote(value) : value}"}
+
+        subquery = if values.size > 1
+          parenthesize values.map(&pair).join("+#{CONNECTIVES[connective]}+")
+        else
+          pair.(values.first)
+        end
+
+        add_subquery subquery
+        self
+      end
+    end
+
+    # Returns the query string.
+    #
+    # @return [String]
+    def to_s
+      @query
+    end
+
+    private
+
+    # Appends a logical connective to the end of the query string.
+    #
+    # @see CONNECTIVES
+    # @param connective [Symbol] The symbol of the logical connective to add.
+    # @return [self]
+    def add_connective(connective)
+      return unless search_query?
+      @query << "+#{CONNECTIVES[connective]}" unless ends_with_connective?
+      self
+    end
+
+    # Appends a subquery to the end of the query string.
+    #
+    # @param subquery [String] The subquery to add.
+    def add_subquery(subquery)
+      if search_query?
+        if ends_with_connective?
+          @query << "+#{subquery}"
+        else
+          add_connective :and
+          @query << "+#{subquery}"
+        end
+      else
+        @query << "&#{PARAMS[:search_query]}=#{subquery}"
+      end
+    end
+
+    # Whether the query string contains the +search_query+ parameter.
+    #
+    # @see PARAMS
+    # @return [Boolean]
+    def search_query?
+      @query.include? PARAMS[:search_query]
+    end
+
+    # Whether the query string ends in a logical connective.
+    #
+    # @see CONNECTIVES
+    # @return [Boolean]
+    def ends_with_connective?
+      CONNECTIVES.values.any? &@query.method(:end_with?)
+    end
+
+    # Parenthesizes a string with CGI-escaped parentheses.
+    #
+    # @param string [String] The string to parenthesize.
+    # @return [String] The parenthesized string.
+    def parenthesize(string)
+      CGI.escape('(') + string + CGI.escape(')')
+    end
+
+    # Enquotes a string with CGI-escaped double quotes.
+    #
+    # @param string [String] The string to enquote.
+    # @return [String] The enquoted string.
+    def enquote(string)
+      CGI.escape("\"") + string + CGI.escape("\"")
+    end
+
+    # Attempt to extract an ID from an arXiv URL.
+    #
+    # @param url [String] The URL to extract the ID from.
+    # @return [String] The extracted ID if successful, otherwise the original string.
+    def extract_id(url)
+      prefix = %r"^(https?\:\/\/)?(www.)?arxiv\.org\/abs\/"
+      if %r"#{prefix}.*$".match? url
+        url.sub(prefix, '').sub(%r"\/$", '')
+      else
+        url
+      end
+    end
+  end
+end
\ No newline at end of file
diff --git a/lib/arx/query/validate.rb b/lib/arx/query/validate.rb
new file mode 100644
index 0000000..0a602c7
--- /dev/null
+++ b/lib/arx/query/validate.rb
@@ -0,0 +1,123 @@
+require_relative '../categories'
+
+module Arx
+
+  # Validations for arXiv search query fields and identifier schemes.
+  module Validate
+
+    # The current arxiv paper identifier scheme (1 April 2007 and onwards).
+    #   The last block of digits can either be five digits (if the paper was published after 1501 - January 2015),
+    #   or four digits (if the paper was published before 1501).
+    #
+    # @see https://arxiv.org/help/arxiv_identifier#new arXiv identifier (new)
+    # @example
+    #   1501.00001
+    #   1705.01662v1
+    #   1412.0135
+    #   0706.0001v2
+    NEW_IDENTIFIER_FORMAT = %r"^\d{4}\.\d{4,5}(v\d+)?$"
+
+    # The legacy arXiv paper identifier scheme (before 1 April 2007).
+    #
+    # @see https://arxiv.org/help/arxiv_identifier#old arXiv identifier (old)
+    # @example
+    #   math/0309136v1
+    #   cond-mat/0211034
+    OLD_IDENTIFIER_FORMAT = %r"^[a-z]+(\-[a-z]+)?\/\d{7}(v\d+)?$"
+
+    class << self
+      # Validates the +sortBy+ field of the query string.
+      #
+      # @param value [Symbol] The value to validate.
+      # @param permitted [Array<Symbol>] Permitted values for the field.
+      # @raise
+      #   [TypeError] If the value is not a +Symbol+.
+      #   [ArgumentError] If the value is not permitted.
+      def sort_by(value, permitted:)
+        raise TypeError.new("Expected `sort_by` to be a Symbol, got: #{value.class}") unless value.is_a? Symbol
+        raise ArgumentError.new("Expected `sort_by` to be one of #{permitted}, got: #{value}") unless permitted.include? value
+      end
+
+      # Validates the +sortOrder+ field of the query string.
+      #
+      # @param value [Symbol] The value to validate.
+      # @param permitted [Array<Symbol>] Permitted values for the field.
+      # @raise
+      #   [TypeError] If the value is not a +Symbol+.
+      #   [ArgumentError] If the value is not permitted.
+      def sort_order(value, permitted:)
+        raise TypeError.new("Expected `sort_order` to be a Symbol, got: #{value.class}") unless value.is_a? Symbol
+        raise ArgumentError.new("Expected `sort_order` to be one of #{permitted}, got: #{value}") unless permitted.include? value
+      end
+
+      # Validates a list of arXiv paper identifiers.
+      #
+      # @param ids [Array<String>] The identifiers to validate.
+      # @raise
+      #   [TypeError] If +ids+ is not an +Array+.
+      #   [TypeError] If any identifier is not a +String+.
+      #   [ArgumentError] If the identifier is invalid.
+      def ids(ids)
+        raise TypeError.new("Expected `ids` to be an Array, got: #{ids.class}") unless ids.is_a? Array
+        ids.each do |id|
+          raise TypeError.new("Expected identifier to be a String, got: #{id.class} (#{id})") unless id.is_a? String
+          raise ArgumentError.new("Malformed arXiv identifier: #{id}") unless id? id
+        end
+      end
+
+      # Validates the +exact+ parameter.
+      #
+      # @param value [Boolean] The value to validate.
+      # @raise
+      #   [TypeError] If the value is not a boolean (+TrueClass+ or +FalseClass+).
+      def exact(value)
+        raise TypeError.new("Expected `exact` to be boolean (TrueClass or FalseClass), got: #{value.class}") unless value == !!value
+      end
+
+      # Validates a logical connective.
+      #
+      # @param value [Symbol] The value to validate.
+      # @param permitted [Array<Symbol>] Permitted values for the field.
+      # @raise
+      #   [TypeError] If the value is not a +Symbol+.
+      #   [ArgumentError] If the value is not permitted.
+      def connective(value, permitted:)
+        raise TypeError.new("Expected `connective` to be a Symbol, got: #{value.class}") unless value.is_a? Symbol
+        raise ArgumentError.new("Expected `connective` to be one of #{permitted}, got: #{value}") unless permitted.include? value
+      end
+
+      # Validates a list of values for the fields of the search query string.
+      #
+      # @param values [Array<String>] The values to validate.
+      # @raise
+      #   [TypeError] If +values+ is not an +Array+.
+      #   [TypeError] If any value is not a +String+.
+      def values(values)
+        raise TypeError.new("Expected `values` to be an Array, got: #{values.class}") unless values.is_a? Array
+        values.each do |value|
+          raise TypeError.new("Expected value to be a String, got: #{value.class} (#{value})") unless value.is_a? String
+        end
+      end
+
+      # Validates a list of arXiv categories.
+      #
+      # @note This is only called after {values}, so there is no need to check types.
+      # @param categories [Array<String>] The categories to validate.
+      # @raise [ArgumentError] If any category is unrecognized (not a valid arXiv category).
+      # @see Arx::CATEGORIES
+      def categories(categories)
+        categories.each do |category|
+          raise ArgumentError.new("Unrecognized arXiv category (#{category}). See Arx::CATEGORIES.") unless Arx::CATEGORIES.keys.include? category
+        end
+      end
+
+      # Validates an arXiv identifier of both the old and new schemes.
+      #
+      # @see NEW_IDENTIFIER_FORMAT
+      # @see OLD_IDENTIFIER_FORMAT
+      def id?(id)
+        NEW_IDENTIFIER_FORMAT.match?(id) || OLD_IDENTIFIER_FORMAT.match?(id)
+      end
+    end
+  end
+end
\ No newline at end of file
diff --git a/lib/arx/version.rb b/lib/arx/version.rb
new file mode 100644
index 0000000..e6b8843
--- /dev/null
+++ b/lib/arx/version.rb
@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+module Arx
+
+  # The current version of Arx.
+  VERSION = {
+    major: 0,
+    minor: 1,
+    patch: 0,
+    meta: nil
+  }.compact.values.join('.').freeze
+end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
new file mode 100644
index 0000000..077279a
--- /dev/null
+++ b/spec/spec_helper.rb
@@ -0,0 +1,12 @@
+$LOAD_PATH.unshift File.join __dir__, '..', 'lib'
+require 'bundler/setup'
+require 'arx'
+
+# Load support files from spec/support
+Dir.glob File.join(__dir__, 'support', '*.rb'), &method(:require)
+
+RSpec.configure do |config|
+  # Configure RSpec here...
+end
+
+include Arx
\ No newline at end of file
diff --git a/spec/support/.gitkeep b/spec/support/.gitkeep
new file mode 100644
index 0000000..e69de29