diff --git a/lib/active_storage/attached/macros.rb b/lib/active_storage/attached/macros.rb
index 96493d121574c..1e0f9a6b7e4ff 100644
--- a/lib/active_storage/attached/macros.rb
+++ b/lib/active_storage/attached/macros.rb
@@ -1,7 +1,18 @@
 module ActiveStorage::Attached::Macros
+  # Specifies the relation between a single attachment and the model.
+  #
+  #   class User < ActiveRecord::Base
+  #     has_one_attached :avatar
+  #   end
+  #
+  # There is no column defined on the model side, Active Storage takes
+  # care of the mapping between your records and the attachment.
+  #
+  # If the +:dependent+ option isn't set, the attachment will be purged
+  # (i.e. destroyed) whenever the record is destroyed.
   def has_one_attached(name, dependent: :purge_later)
     define_method(name) do
-      instance_variable_get("@active_storage_attached_#{name}") || 
+      instance_variable_get("@active_storage_attached_#{name}") ||
         instance_variable_set("@active_storage_attached_#{name}", ActiveStorage::Attached::One.new(name, self))
     end
 
@@ -10,9 +21,20 @@ def has_one_attached(name, dependent: :purge_later)
     end
   end
 
+  # Specifies the relation between multiple attachments and the model.
+  #
+  #   class Gallery < ActiveRecord::Base
+  #     has_many_attached :photos
+  #   end
+  #
+  # There are no columns defined on the model side, Active Storage takes
+  # care of the mapping between your records and the attachments.
+  #
+  # If the +:dependent+ option isn't set, all the attachments will be purged
+  # (i.e. destroyed) whenever the record is destroyed.
   def has_many_attached(name, dependent: :purge_later)
     define_method(name) do
-      instance_variable_get("@active_storage_attached_#{name}") || 
+      instance_variable_get("@active_storage_attached_#{name}") ||
         instance_variable_set("@active_storage_attached_#{name}", ActiveStorage::Attached::Many.new(name, self))
     end
 
diff --git a/lib/active_storage/attached/many.rb b/lib/active_storage/attached/many.rb
index f1535dfbc6187..99d980196a1a8 100644
--- a/lib/active_storage/attached/many.rb
+++ b/lib/active_storage/attached/many.rb
@@ -1,20 +1,36 @@
+# Representation of multiple attachments to a model.
 class ActiveStorage::Attached::Many < ActiveStorage::Attached
   delegate_missing_to :attachments
 
+  # Returns all the associated attachment records.
+  #
+  # You don't have to call this method to access the attachments' methods as
+  # they are all available at the model level.
   def attachments
     @attachments ||= ActiveStorage::Attachment.where(record_gid: record.to_gid.to_s, name: name)
   end
 
+  # Associates one or several attachments with the current record, saving
+  # them to the database.
   def attach(*attachables)
     @attachments = attachments | Array(attachables).flatten.collect do |attachable|
       ActiveStorage::Attachment.create!(record_gid: record.to_gid.to_s, name: name, blob: create_blob_from(attachable))
     end
   end
 
+  # Checks the presence of attachments.
+  #
+  #   class Gallery < ActiveRecord::Base
+  #     has_many_attached :photos
+  #   end
+  #
+  #   Gallery.new.photos.attached? # => false
   def attached?
     attachments.any?
   end
 
+  # Directly purges each associated attachment (i.e. destroys the blobs and
+  # attachments and deletes the files on the service).
   def purge
     if attached?
       attachments.each(&:purge)
@@ -22,6 +38,7 @@ def purge
     end
   end
 
+  # Purges each associated attachment through the queuing system.
   def purge_later
     if attached?
       attachments.each(&:purge_later)
diff --git a/lib/active_storage/attached/one.rb b/lib/active_storage/attached/one.rb
index d08d265992a87..80e4cb623489c 100644
--- a/lib/active_storage/attached/one.rb
+++ b/lib/active_storage/attached/one.rb
@@ -1,18 +1,34 @@
+# Representation of a single attachment to a model.
 class ActiveStorage::Attached::One < ActiveStorage::Attached
   delegate_missing_to :attachment
 
+  # Returns the associated attachment record.
+  #
+  # You don't have to call this method to access the attachment's methods as
+  # they are all available at the model level.
   def attachment
     @attachment ||= ActiveStorage::Attachment.find_by(record_gid: record.to_gid.to_s, name: name)
   end
 
+  # Associates a given attachment with the current record, saving it to the
+  # database.
   def attach(attachable)
     @attachment = ActiveStorage::Attachment.create!(record_gid: record.to_gid.to_s, name: name, blob: create_blob_from(attachable))
   end
 
+  # Checks the presence of the attachment.
+  #
+  #   class User < ActiveRecord::Base
+  #     has_one_attached :avatar
+  #   end
+  #
+  #   User.new.avatar.attached? # => false
   def attached?
     attachment.present?
   end
 
+  # Directly purges the attachment (i.e. destroys the blob and
+  # attachment and deletes the file on the service).
   def purge
     if attached?
       attachment.purge
@@ -20,6 +36,7 @@ def purge
     end
   end
 
+  # Purges the attachment through the queuing system.
   def purge_later
     if attached?
       attachment.purge_later
diff --git a/lib/active_storage/disk_controller.rb b/lib/active_storage/disk_controller.rb
index 3eba86c2137aa..9d5b52d66f95a 100644
--- a/lib/active_storage/disk_controller.rb
+++ b/lib/active_storage/disk_controller.rb
@@ -4,11 +4,21 @@
 
 require "active_support/core_ext/object/inclusion"
 
+# This controller is a wrapper around local file downloading. It allows you to
+# make abstraction of the URL generation logic and to serve files with expiry
+# if you are using the +Disk+ service.
+#
+# By default, mounting the Active Storage engine inside your application will
+# define a +/rails/blobs/:encoded_key+ route that will reference this controller's
+# +show+ action and will be used to serve local files.
+#
+# A URL for an attachment can be generated through its +#url+ method, that
+# will use the aforementioned route.
 class ActiveStorage::DiskController < ActionController::Base
   def show
     if key = decode_verified_key
       blob = ActiveStorage::Blob.find_by!(key: key)
-      
+
       if stale?(etag: blob.checksum)
         send_data blob.download, filename: blob.filename, type: blob.content_type, disposition: disposition_param
       end
diff --git a/lib/active_storage/migration.rb b/lib/active_storage/migration.rb
index 433dd5026feb0..c56e7a1786c8d 100644
--- a/lib/active_storage/migration.rb
+++ b/lib/active_storage/migration.rb
@@ -1,4 +1,4 @@
-class ActiveStorageCreateTables < ActiveRecord::Migration[5.1]
+class ActiveStorageCreateTables < ActiveRecord::Migration[5.1] # :nodoc:
   def change
     create_table :active_storage_blobs do |t|
       t.string   :key
diff --git a/lib/active_storage/service.rb b/lib/active_storage/service.rb
index 9aab654d80d90..f15958fda94de 100644
--- a/lib/active_storage/service.rb
+++ b/lib/active_storage/service.rb
@@ -1,4 +1,34 @@
 # Abstract class serving as an interface for concrete services.
+#
+# The available services are:
+#
+# * +Disk+, to manage attachments saved directly on the hard drive.
+# * +GCS+, to manage attachments through Google Cloud Storage.
+# * +S3+, to manage attachments through Amazon S3.
+# * +Mirror+, to be able to use several services to manage attachments.
+#
+# Inside a Rails application, you can set-up your services through the
+# generated <tt>config/storage_services.yml</tt> file and reference one
+# of the aforementioned constant under the +service+ key. For example:
+#
+#   local:
+#     service: Disk
+#     root: <%= Rails.root.join("storage") %>
+#
+# You can checkout the service's constructor to know which keys are required.
+#
+# Then, in your application's configuration, you can specify the service to
+# use like this:
+#
+#   config.active_storage.service = :local
+#
+# If you are using Active Storage outside of a Ruby on Rails application, you
+# can configure the service to use like this:
+#
+#   ActiveStorage::Blob.service = ActiveStorage::Service.configure(
+#     :Disk,
+#     root: Pathname("/foo/bar/storage")
+#   )
 class ActiveStorage::Service
   class ActiveStorage::IntegrityError < StandardError; end
 
@@ -11,7 +41,6 @@ def self.configure(service, **options)
     end
   end
 
-
   def upload(key, io, checksum: nil)
     raise NotImplementedError
   end