diff --git a/.rubocop.yml b/.rubocop.yml
index 1c90cf3..d5b4ba0 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -5,6 +5,7 @@ AllCops:
     - "bin/**/*"
     - "Gemfile"
     - "Rakefile"
+  SuggestExtensions: false
 
 Metrics/BlockLength:
   Exclude:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index fa347af..2149319 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 # Changelog
 
+## 0.2.0
+
+- Add `OptionsDeclaration` mixin.
+
 ## 0.1.1
 
 ### Bug fixes
diff --git a/Gemfile.lock b/Gemfile.lock
index 07a0fd5..f14251a 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -2,12 +2,23 @@ PATH
   remote: .
   specs:
     stimpack (0.1.0)
+      activesupport (~> 6.1)
 
 GEM
   remote: https://rubygems.org/
   specs:
+    activesupport (6.1.3)
+      concurrent-ruby (~> 1.0, >= 1.0.2)
+      i18n (>= 1.6, < 2)
+      minitest (>= 5.1)
+      tzinfo (~> 2.0)
+      zeitwerk (~> 2.3)
     ast (2.4.2)
+    concurrent-ruby (1.1.8)
     diff-lcs (1.4.4)
+    i18n (1.8.9)
+      concurrent-ruby (~> 1.0)
+    minitest (5.14.4)
     parallel (1.20.1)
     parser (3.0.0.0)
       ast (~> 2.4.1)
@@ -40,7 +51,10 @@ GEM
     rubocop-ast (1.4.1)
       parser (>= 2.7.1.5)
     ruby-progressbar (1.11.0)
+    tzinfo (2.0.4)
+      concurrent-ruby (~> 1.0)
     unicode-display_width (2.0.0)
+    zeitwerk (2.4.2)
 
 PLATFORMS
   x86_64-darwin-19
diff --git a/README.md b/README.md
index eab2182..b00f4ed 100644
--- a/README.md
+++ b/README.md
@@ -8,6 +8,7 @@ and behaviour.
 ## Table of Contents
 
 - [FunctionalObject](#functionalobject)
+- [OptionsDeclaration](#optionsdeclaration)
 
 ## FunctionalObject
 
@@ -42,3 +43,44 @@ we can now initialize and invoke an instance of `Foo` by calling:
 Foo.(bar: "Hello world!")
 #=> "Hello world!"
 ```
+
+## OptionsDeclaration
+
+A mixin that introduces the concept of an `option`, and lets classes declare
+a list options with various configuration options. Declaring an option will:
+
+1. Add a keyword argument to the class initializer.
+2. Assign an instance variable on instantiation.
+3. Create an attribute reader (private by default.)
+
+This lets us collect and condense what would otherwise be scattered throughout
+the class definition.
+
+**Example:**
+
+Given the following options declaration:
+
+```ruby
+class Foo
+  include Stimpack::OptionsDeclaration
+
+  option :bar
+  option :baz, default: []
+end
+```
+
+we can now instantiate `Foo` as long as we provide the required options:
+
+```ruby
+Foo.new(bar: "Hello!")
+```
+
+### Configuration options
+
+When declaring an option, the following configuration kets are available:
+
+| Configuration    | Type         | Default | Notes |
+| ---------------  | ------------ | ------- | ----- |
+| `default`        | `any`        | `nil`   | Can be a literal or a callable object. Arrays and hashes will not be shared across instances. |
+| `required`       | `boolean`    | `true`  | |
+| `private_reader` | `boolean`    | `true`  | |
diff --git a/lib/stimpack.rb b/lib/stimpack.rb
index 5a8716f..fb374b5 100644
--- a/lib/stimpack.rb
+++ b/lib/stimpack.rb
@@ -3,6 +3,7 @@
 require_relative "stimpack/version"
 
 require_relative "stimpack/functional_object"
+require_relative "stimpack/options_declaration"
 
 module Stimpack
   class Error < StandardError; end
diff --git a/lib/stimpack/options_declaration.rb b/lib/stimpack/options_declaration.rb
new file mode 100644
index 0000000..f58dbf7
--- /dev/null
+++ b/lib/stimpack/options_declaration.rb
@@ -0,0 +1,170 @@
+# frozen_string_literal: true
+
+# TODO: Remove dependency on ActiveSupport.
+#
+require "active_support/core_ext/class/attribute"
+
+module Stimpack
+  # This mixin is used to augment classes with a DSL for declaring keyword
+  # arguments. It is used to cut down on noise for a common initialization
+  # pattern, which goes:
+  #
+  #   1. Declare parameters in `#initialize`.
+  #   2. Assign attributes to instance variables.
+  #   3. Add private reader methods.
+  #
+  # Example:
+  #
+  #   # Before
+  #
+  #   class AccruePoints
+  #     def initialize(user:, amount:)
+  #       @user = user
+  #       @amount = amount
+  #     end
+  #
+  #     private
+  #
+  #     attr_reader :user, :amount
+  #   end
+  #
+  #   # After
+  #
+  #   class AccruePoints
+  #     option :user
+  #     option :amount
+  #   end
+  #
+  module OptionsDeclaration
+    module ClassMethods
+      def self.extended(klass)
+        klass.class_eval do
+          # TODO: Remove dependency on ActiveSupport.
+          #
+          class_attribute :options_configuration, instance_accessor: false, default: {}
+        end
+      end
+
+      # Declare a keyword argument for this class.
+      #
+      # Example:
+      #
+      #   class AccruePoints
+      #     option :user
+      #   end
+      #
+      def option(*identifiers, required: true, default: nil, private_reader: true) # rubocop:disable Metrics/MethodLength
+        self.options_configuration = options_configuration.merge(
+          identifiers.map do |identifier|
+            [
+              identifier.to_sym,
+              Option.new(
+                identifier.to_sym,
+                required: required,
+                default: default
+              )
+            ]
+          end.to_h
+        )
+
+        identifiers.each do |identifier|
+          class_eval do
+            attr_reader identifier.to_sym
+
+            private identifier.to_sym if private_reader
+          end
+        end
+      end
+
+      def options
+        options_configuration.keys
+      end
+
+      def required_options
+        options_configuration.select { |_, option| option.required? }.keys
+      end
+
+      def optional_options
+        options_configuration.select { |_, option| option.optional? }.keys
+      end
+    end
+
+    # Injects an initializer that assigns options and proxies the call to any
+    # custom initializer _without_ the declared options included in the call.
+    #
+    module OptionsInitializer
+      def initialize(*_args, **options)
+        assigner = OptionsAssigner.new(self, options)
+        assigner.assign_options!
+        yield self if block_given?
+      end
+
+      class OptionsAssigner
+        def initialize(service, options)
+          @service = service
+          @options = options
+        end
+
+        def assign_options!
+          check_for_missing_options!
+
+          service.class.options_configuration.each_value { |o| assign_option(o) }
+        end
+
+        private
+
+        attr_reader :service, :options
+
+        def check_for_missing_options!
+          raise(ArgumentError, <<~ERROR) unless missing_options.empty?
+            Missing required options: #{missing_options.join(', ')}
+          ERROR
+        end
+
+        def assign_option(option)
+          assigned_value = options[option.name]
+
+          service.instance_variable_set(
+            "@#{option.name}",
+            assigned_value.nil? ? option.default_value : assigned_value
+          )
+        end
+
+        def missing_options
+          required_options - options.keys
+        end
+
+        def required_options
+          service.class.required_options
+        end
+      end
+    end
+
+    def self.included(klass)
+      klass.extend(ClassMethods)
+      klass.include(OptionsInitializer)
+    end
+
+    class Option
+      def initialize(name, required:, default:)
+        @name = name
+        @default = default
+        @required = required
+      end
+
+      attr_reader :name, :default, :required
+
+      def default_value
+        default.respond_to?(:call) ? default.() : default
+      end
+
+      def required?
+        required && default.nil?
+      end
+
+      def optional?
+        !required?
+      end
+    end
+  end
+end
diff --git a/spec/stimpack/options_declaration_spec.rb b/spec/stimpack/options_declaration_spec.rb
new file mode 100644
index 0000000..b4081d2
--- /dev/null
+++ b/spec/stimpack/options_declaration_spec.rb
@@ -0,0 +1,124 @@
+# frozen_string_literal: true
+
+require "stimpack/options_declaration"
+
+RSpec.describe Stimpack::OptionsDeclaration do
+  subject(:service) { klass }
+
+  let(:klass) do
+    Class.new do
+      include Stimpack::OptionsDeclaration
+
+      option :foo
+      option :bar, required: false
+      option :baz, private_reader: false
+      option :qux, default: "Foo"
+      option :quux, default: -> { "Bar" }
+    end
+  end
+
+  describe ".option" do
+    it { expect(service.options_configuration.size).to eq(5) }
+    it { expect(service.options_configuration.values).to all(be_a(described_class::Option)) }
+
+    describe "private_reader (option)" do
+      let(:public_instance_methods) { service.public_instance_methods(false) }
+      let(:private_instance_methods) { service.private_instance_methods(false) }
+
+      it { expect(public_instance_methods).to contain_exactly(:baz) }
+      it { expect(private_instance_methods).to contain_exactly(:foo, :bar, :qux, :quux) }
+    end
+  end
+
+  describe ".options" do
+    it { expect(service.options).to contain_exactly(:foo, :bar, :baz, :qux, :quux) }
+  end
+
+  describe ".required_options" do
+    it { expect(service.required_options).to contain_exactly(:foo, :baz) }
+  end
+
+  describe ".optional_options" do
+    it { expect(service.optional_options).to contain_exactly(:bar, :qux, :quux) }
+  end
+
+  describe "#initialize" do
+    context "when not passing required options" do
+      it { expect { service.new }.to raise_error(ArgumentError) }
+    end
+
+    context "when omitting only optional options" do
+      let(:options) do
+        {
+          foo: 1,
+          baz: 2
+        }
+      end
+
+      it { expect { service.new(**options) }.not_to raise_error }
+    end
+
+    describe "argument assignment" do
+      let(:instance) { service.new(**options) }
+
+      let(:options) do
+        {
+          foo: 1,
+          baz: 2
+        }
+      end
+
+      context "when option is assigned explicitly" do
+        it { expect(instance.send(:foo)).to eq(1) }
+        it { expect(instance.send(:baz)).to eq(2) }
+      end
+
+      context "when option with default is assigned explicitly" do
+        before { options.merge!(qux: 3) }
+
+        it { expect(instance.send(:qux)).to eq(3) }
+      end
+
+      context "when optional option is assigned through omission" do
+        it { expect(instance.send(:bar)).to eq(nil) }
+      end
+
+      context "when default option is assigned by omission" do
+        it { expect(instance.send(:qux)).to eq("Foo") }
+        it { expect(instance.send(:quux)).to eq("Bar") }
+      end
+    end
+  end
+
+  describe "#initialize (multiple-layer inheritance)" do
+    let(:sub_klass) do
+      Class.new(klass) do
+        # Override option :foo and add 2 more options.
+        #
+        option :lorem, :ipsum, :foo, private_reader: false
+
+        attr_accessor :hello, :world, :qux
+
+        def initialize(hello, world:, **options)
+          super(**options)
+          @hello = hello
+          @world = world
+        end
+      end
+    end
+
+    let(:instance) do
+      sub_klass.new("hello", world: "world", foo: 1, baz: 2, lorem: 3, ipsum: 4) do |instance|
+        instance.qux = instance.lorem + instance.ipsum
+      end
+    end
+
+    it { expect(instance.hello).to eq("hello") }
+    it { expect(instance.world).to eq("world") }
+    it { expect(instance.foo).to eq(1) }
+    it { expect(instance.baz).to eq(2) }
+    it { expect(instance.lorem).to eq(3) }
+    it { expect(instance.ipsum).to eq(4) }
+    it { expect(instance.qux).to eq(7) }
+  end
+end
diff --git a/stimpack.gemspec b/stimpack.gemspec
index 52b9827..923c59b 100644
--- a/stimpack.gemspec
+++ b/stimpack.gemspec
@@ -29,6 +29,10 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{\Abin/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
+  # TODO: Drop ActiveSupport dependency.
+  #
+  spec.add_dependency "activesupport", "~> 6.1"
+
   spec.add_development_dependency "rspec", "~> 3.10"
   spec.add_development_dependency "rubocop", "~> 1.11"
 end