diff --git a/lib/global_id/identification.rb b/lib/global_id/identification.rb
index 91bc611..1cd4941 100644
--- a/lib/global_id/identification.rb
+++ b/lib/global_id/identification.rb
@@ -1,19 +1,118 @@
 class GlobalID
+  # Mix `GlobalID::Identification` into any model with a `#find(id)` class
+  # method. Support is automatically included in Active Record.
+  #
+  #   class Person
+  #     include ActiveModel::Model
+  #     include GlobalID::Identification
+  #
+  #     attr_accessor :id
+  #
+  #     def self.find(id)
+  #       new id: id
+  #     end
+  #
+  #     def ==(other)
+  #       id == other.try(:id)
+  #     end
+  #   end
+  #
+  #   person_gid = Person.find(1).to_global_id
+  #   # => #<GlobalID ...
+  #   person_gid.uri
+  #   # => #<URI ...
+  #   person_gid.to_s
+  #   # => "gid://app/Person/1"
+  #   GlobalID::Locator.locate person_gid
+  #   # => #<Person:0x007fae94bf6298 @id="1">
   module Identification
+
+    # Returns the Global ID of the model.
+    #
+    #   model = Person.new id: 1
+    #   global_id = model.to_global_id
+    #   global_id.modal_class # => Person
+    #   global_id.modal_id # => "1"
+    #   global_id.to_param # => "Z2lkOi8vYm9yZGZvbGlvL1BlcnNvbi8x"
     def to_global_id(options = {})
       GlobalID.create(self, options)
     end
     alias to_gid to_global_id
 
+    # Returns the Global ID parameter of the model.
+    #
+    #   model = Person.new id: 1
+    #   model.to_gid_param # => ""Z2lkOi8vYm9yZGZvbGlvL1BlcnNvbi8x"
     def to_gid_param(options = {})
       to_global_id(options).to_param
     end
 
+    # Returns the Signed Global ID of the model.
+    # Signed Global IDs ensure that the data hasn't been tampered with.
+    #
+    #   model = Person.new id: 1
+    #   signed_global_id = model.to_signed_global_id
+    #   signed_global_id.modal_class # => Person
+    #   signed_global_id.modal_id # => "1"
+    #   signed_global_id.to_param # => "BAh7CEkiCGdpZAY6BkVUSSIiZ2..."
+    #
+    # ==== Expiration
+    #
+    # Signed Global IDs can expire some time in the future. This is useful if
+    # there's a resource people shouldn't have indefinite access to, like a
+    # share link.
+    #
+    #   expiring_sgid = Document.find(5).to_sgid(expires_in: 2.hours, for: 'sharing')
+    #   # => #<SignedGlobalID:0x008fde45df8937 ...>
+    #   # Within 2 hours...
+    #   GlobalID::Locator.locate_signed(expiring_sgid.to_s, for: 'sharing')
+    #   # => #<Document:0x007fae94bf6298 @id="5">
+    #   # More than 2 hours later...
+    #   GlobalID::Locator.locate_signed(expiring_sgid.to_s, for: 'sharing')
+    #   # => nil
+    #
+    # In Rails, an auto-expiry of 1 month is set by default.
+    #
+    # You need to explicitly pass `expires_in: nil` to generate a permanent
+    # SGID that will not expire,
+    #
+    #   never_expiring_sgid = Document.find(5).to_sgid(expires_in: nil)
+    #   # => #<SignedGlobalID:0x008fde45df8937 ...>
+    #
+    #   # Any time later...
+    #   GlobalID::Locator.locate_signed never_expiring_sgid
+    #   # => #<Document:0x007fae94bf6298 @id="5">
+    #
+    # It's also possible to pass a specific expiry time
+    #
+    #   explicit_expiring_sgid = SecretAgentMessage.find(5).to_sgid(expires_at: Time.now.advance(hours: 1))
+    #   # => #<SignedGlobalID:0x008fde45df8937 ...>
+    #
+    #   # 1 hour later...
+    #   GlobalID::Locator.locate_signed explicit_expiring_sgid.to_s
+    #   # => nil
+    #
+    # Note that an explicit `:expires_at` takes precedence over a relative `:expires_in`.
+    #
+    # ==== Purpose
+    #
+    # You can even bump the security up some more by explaining what purpose a
+    # Signed Global ID is for. In this way evildoers can't reuse a sign-up
+    # form's SGID on the login page. For example.
+    #
+    #   signup_person_sgid = Person.find(1).to_sgid(for: 'signup_form')
+    #   # => #<SignedGlobalID:0x007fea1984b520
+    #   GlobalID::Locator.locate_signed(signup_person_sgid.to_s, for: 'signup_form')
+    #   => #<Person:0x007fae94bf6298 @id="1">
     def to_signed_global_id(options = {})
       SignedGlobalID.create(self, options)
     end
     alias to_sgid to_signed_global_id
 
+    # Returns the Signed Global ID parameter.
+    #
+    #   model = Person.new id: 1
+    #   model.to_sgid_param # => "BAh7CEkiCGdpZAY6BkVUSSIiZ2..."
     def to_sgid_param(options = {})
       to_signed_global_id(options).to_param
     end