diff --git a/README.md b/README.md
index df1e7c5f..3691dc5f 100644
--- a/README.md
+++ b/README.md
@@ -45,6 +45,7 @@ has been destroyed.
   - [5.a. Single Table Inheritance](#5a-single-table-inheritance-sti)
   - [5.b. Configuring the `versions` Association](#5b-configuring-the-versions-association)
   - [5.c. Generators](#5c-generators)
+  - [5.d. Protected Attributes](#5d-protected-attributes)
 - [6. Extensibility](#6-extensibility)
   - [6.a. Custom Version Classes](#6a-custom-version-classes)
   - [6.b. Custom Serializer](#6b-custom-serializer)
@@ -1008,30 +1009,12 @@ class ApplicationController
 end
 ```
 
-#### Protected Attributes and Metadata
-
-If you are using rails 3 or the [protected_attributes][17] gem you must declare
-your metadata columns to be `attr_accessible`.
-
-```ruby
-# app/models/paper_trail/version.rb
-module PaperTrail
-  class Version < ActiveRecord::Base
-    include PaperTrail::VersionConcern
-    attr_accessible :author_id, :word_count, :answer
-  end
-end
-```
-
-If you're using [strong_parameters][18] instead of [protected_attributes][17]
-then there is no need to use `attr_accessible`.
-
 ## 5. ActiveRecord
 
 ### 5.a. Single Table Inheritance (STI)
 
 PaperTrail supports [Single Table Inheritance][39], and even supports an
-un-versioned base model, as of 23ffbdc7e1.
+un-versioned base model, as of `23ffbdc7e1`.
 
 ```ruby
 class Fruit < ActiveRecord::Base
@@ -1087,6 +1070,12 @@ Runtime options:
 Generates (but does not run) a migration to add a versions table.  Also generates an initializer file for configuring PaperTrail
 ```
 
+### 5.d. Protected Attributes
+
+As of version 6, PT no longer supports rails 3 or the [protected_attributes][17]
+gem. If you are still using them, you may use PT 5 or lower. We recommend
+upgrading to [strong_parameters][18] as soon as possible.
+
 ## 6. Extensibility
 
 ### 6.a. Custom Version Classes
diff --git a/lib/paper_trail.rb b/lib/paper_trail.rb
index bbe9225f..e405a3eb 100644
--- a/lib/paper_trail.rb
+++ b/lib/paper_trail.rb
@@ -121,15 +121,6 @@ module PaperTrail
       PaperTrail.config.serializer
     end
 
-    # Returns a boolean indicating whether "protected attibutes" should be
-    # configured, e.g. attr_accessible, mass_assignment_sanitizer,
-    # whitelist_attributes, etc.
-    # @api public
-    def active_record_protected_attributes?
-      @active_record_protected_attributes ||= ::ActiveRecord::VERSION::MAJOR < 4 ||
-        !!defined?(ProtectedAttributes)
-    end
-
     # @api public
     def transaction?
       ::ActiveRecord::Base.connection.open_transactions > 0
@@ -167,17 +158,6 @@ module PaperTrail
   end
 end
 
-# If available, ensure that the `protected_attributes` gem is loaded
-# before the `Version` class.
-unless PaperTrail.active_record_protected_attributes?
-  PaperTrail.send(:remove_instance_variable, :@active_record_protected_attributes)
-  begin
-    require "protected_attributes"
-  rescue LoadError # rubocop:disable Lint/HandleExceptions
-    # In case `protected_attributes` gem is not available.
-  end
-end
-
 ActiveSupport.on_load(:active_record) do
   include PaperTrail::Model
 end
diff --git a/lib/paper_trail/frameworks/rails/controller.rb b/lib/paper_trail/frameworks/rails/controller.rb
index c7854e5f..8450ee97 100644
--- a/lib/paper_trail/frameworks/rails/controller.rb
+++ b/lib/paper_trail/frameworks/rails/controller.rb
@@ -4,24 +4,12 @@ module PaperTrail
     # information to the model layer, with `controller_info` and `whodunnit`.
     # Also includes a convenient on/off switch, `enabled_for_controller`.
     module Controller
-      def self.included(base)
-        before = [
+      def self.included(controller)
+        controller.before_action(
           :set_paper_trail_enabled_for_controller,
           :set_paper_trail_controller_info
-        ]
-        after = [
-          :warn_about_not_setting_whodunnit
-        ]
-
-        if base.respond_to? :before_action
-          # Rails 4+
-          before.map { |sym| base.before_action sym }
-          after.map  { |sym| base.after_action  sym }
-        else
-          # Rails 3.
-          before.map { |sym| base.before_filter sym }
-          after.map  { |sym| base.after_filter  sym }
-        end
+        )
+        controller.after_action :warn_about_not_setting_whodunnit
       end
 
       protected
diff --git a/lib/paper_trail/version_association_concern.rb b/lib/paper_trail/version_association_concern.rb
index e94ac246..5fee7bb0 100644
--- a/lib/paper_trail/version_association_concern.rb
+++ b/lib/paper_trail/version_association_concern.rb
@@ -8,10 +8,6 @@ module PaperTrail
 
     included do
       belongs_to :version
-
-      if PaperTrail.active_record_protected_attributes?
-        attr_accessible :version_id, :foreign_key_name, :foreign_key_id
-      end
     end
   end
 end
diff --git a/lib/paper_trail/version_concern.rb b/lib/paper_trail/version_concern.rb
index 25e45863..3f67d39b 100644
--- a/lib/paper_trail/version_concern.rb
+++ b/lib/paper_trail/version_concern.rb
@@ -21,22 +21,7 @@ module PaperTrail
       end
 
       validates_presence_of :event
-
-      if PaperTrail.active_record_protected_attributes?
-        attr_accessible(
-          :item_type,
-          :item_id,
-          :event,
-          :whodunnit,
-          :object,
-          :object_changes,
-          :transaction_id,
-          :created_at
-        )
-      end
-
       after_create :enforce_version_limit!
-
       scope :within_transaction, ->(id) { where transaction_id: id }
     end
 
@@ -225,9 +210,7 @@ module PaperTrail
     #
     def reify(options = {})
       return nil if object.nil?
-      without_identity_map do
-        ::PaperTrail::Reifier.reify(self, options)
-      end
+      ::PaperTrail::Reifier.reify(self, options)
     end
 
     # Returns what changed in this version of the item.
@@ -323,17 +306,6 @@ module PaperTrail
       end
     end
 
-    # In Rails 3.1+, calling reify on a previous version confuses the
-    # IdentityMap, if enabled. This prevents insertion into the map.
-    # @api private
-    def without_identity_map(&block)
-      if defined?(::ActiveRecord::IdentityMap) && ::ActiveRecord::IdentityMap.respond_to?(:without)
-        ::ActiveRecord::IdentityMap.without(&block)
-      else
-        yield
-      end
-    end
-
     # Checks that a value has been set for the `version_limit` config
     # option, and if so enforces it.
     # @api private
diff --git a/test/dummy/app/controllers/application_controller.rb b/test/dummy/app/controllers/application_controller.rb
index 2ab0533a..c5d48b9c 100644
--- a/test/dummy/app/controllers/application_controller.rb
+++ b/test/dummy/app/controllers/application_controller.rb
@@ -1,18 +1,15 @@
 class ApplicationController < ActionController::Base
   protect_from_forgery
 
-  # Rails 5 deprecates `before_filter`
-  name_of_before_callback = respond_to?(:before_action) ? :before_action : :before_filter
-
   # Some applications and libraries modify `current_user`. Their changes need
   # to be reflected in `whodunnit`, so the `set_paper_trail_whodunnit` below
   # must happen after this.
-  send(name_of_before_callback, :modify_current_user)
+  before_action :modify_current_user
 
-  # Going forward, we'll no longer add this `before_filter`, requiring people
-  # to do so themselves, allowing them to control the order in which this filter
-  # happens.
-  send(name_of_before_callback, :set_paper_trail_whodunnit)
+  # PT used to add this callback automatically. Now people are required to add
+  # it themsevles, like this, allowing them to control the order of callbacks.
+  # The `modify_current_user` callback above shows why this control is useful.
+  before_action :set_paper_trail_whodunnit
 
   def rescue_action(e)
     raise e
diff --git a/test/dummy/app/controllers/articles_controller.rb b/test/dummy/app/controllers/articles_controller.rb
index fb671296..e7a9f5d0 100644
--- a/test/dummy/app/controllers/articles_controller.rb
+++ b/test/dummy/app/controllers/articles_controller.rb
@@ -11,10 +11,6 @@ class ArticlesController < ApplicationController
   private
 
   def article_params
-    if PaperTrail.active_record_protected_attributes?
-      params[:article]
-    else
-      params.require(:article).permit!
-    end
+    params.require(:article).permit!
   end
 end
diff --git a/test/dummy/app/controllers/widgets_controller.rb b/test/dummy/app/controllers/widgets_controller.rb
index 0b9648e4..1fc58630 100644
--- a/test/dummy/app/controllers/widgets_controller.rb
+++ b/test/dummy/app/controllers/widgets_controller.rb
@@ -23,10 +23,6 @@ class WidgetsController < ApplicationController
   private
 
   def widget_params
-    if PaperTrail.active_record_protected_attributes?
-      params[:widget]
-    else
-      params[:widget].permit!
-    end
+    params[:widget].permit!
   end
 end
diff --git a/test/dummy/app/models/animal.rb b/test/dummy/app/models/animal.rb
index 8411f2ae..ef2629bc 100644
--- a/test/dummy/app/models/animal.rb
+++ b/test/dummy/app/models/animal.rb
@@ -1,6 +1,4 @@
 class Animal < ActiveRecord::Base
   has_paper_trail
   self.inheritance_column = "species"
-
-  attr_accessible :species, :name if ::PaperTrail.active_record_protected_attributes?
 end
diff --git a/test/dummy/app/models/protected_widget.rb b/test/dummy/app/models/protected_widget.rb
deleted file mode 100644
index 7a5f6fb1..00000000
--- a/test/dummy/app/models/protected_widget.rb
+++ /dev/null
@@ -1,3 +0,0 @@
-class ProtectedWidget < Widget
-  attr_accessible :name, :a_text if ::PaperTrail.active_record_protected_attributes?
-end
diff --git a/test/dummy/config/application.rb b/test/dummy/config/application.rb
index c16ae612..f4511be2 100644
--- a/test/dummy/config/application.rb
+++ b/test/dummy/config/application.rb
@@ -45,14 +45,6 @@ module Dummy
     # like if you have constraints or database-specific column types
     # config.active_record.schema_format = :sql
 
-    # Enforce whitelist mode for mass assignment.
-    # This will create an empty whitelist of attributes available for mass-assignment for all models
-    # in your app. As such, your models will need to explicitly whitelist or blacklist accessible
-    # parameters by using an attr_accessible or attr_protected declaration.
-    if ::PaperTrail.active_record_protected_attributes?
-      config.active_record.whitelist_attributes = false
-    end
-
     # `config.assets` is a `NoMethodError` in rails 5.
     config.assets.enabled = false if config.respond_to?(:assets)
 
diff --git a/test/dummy/config/environments/development.rb b/test/dummy/config/environments/development.rb
index 0604cccd..38dafe17 100644
--- a/test/dummy/config/environments/development.rb
+++ b/test/dummy/config/environments/development.rb
@@ -22,11 +22,6 @@ Dummy::Application.configure do
   # Only use best-standards-support built into browsers
   config.action_dispatch.best_standards_support = :builtin
 
-  # Raise exception on mass assignment protection for Active Record models
-  if ::PaperTrail.active_record_protected_attributes?
-    config.active_record.mass_assignment_sanitizer = :strict
-  end
-
   # Log the query plan for queries taking more than this (works
   # with SQLite, MySQL, and PostgreSQL)
   # config.active_record.auto_explain_threshold_in_seconds = 0.5
diff --git a/test/dummy/config/environments/test.rb b/test/dummy/config/environments/test.rb
index dd4d9ae7..eeddff31 100644
--- a/test/dummy/config/environments/test.rb
+++ b/test/dummy/config/environments/test.rb
@@ -41,11 +41,6 @@ Dummy::Application.configure do
   # ActionMailer::Base.deliveries array.
   # config.action_mailer.delivery_method = :test
 
-  # Raise exception on mass assignment protection for Active Record models
-  if ::PaperTrail.active_record_protected_attributes?
-    config.active_record.mass_assignment_sanitizer = :strict
-  end
-
   # Print deprecation notices to the stderr
   config.active_support.deprecation = :stderr
 end
diff --git a/test/dummy/config/initializers/paper_trail.rb b/test/dummy/config/initializers/paper_trail.rb
index 815cc748..4af1ebab 100644
--- a/test/dummy/config/initializers/paper_trail.rb
+++ b/test/dummy/config/initializers/paper_trail.rb
@@ -1,9 +1 @@
 PaperTrail.config.track_associations = true
-
-module PaperTrail
-  class Version < ActiveRecord::Base
-    if ::PaperTrail.active_record_protected_attributes?
-      attr_accessible :answer, :action, :question, :article_id, :ip, :user_agent, :title
-    end
-  end
-end
diff --git a/test/dummy/script/rails b/test/dummy/script/rails
index 2d248d55..57c4afb3 100755
--- a/test/dummy/script/rails
+++ b/test/dummy/script/rails
@@ -1,8 +1,4 @@
 #!/usr/bin/env ruby
-
-# This command will automatically be run when you run "rails" with Rails 3 gems
-# installed from the root of your application.
-
 APP_PATH = File.expand_path("../../config/application", __FILE__)
 require File.expand_path("../../config/boot", __FILE__)
 require "rails/commands"
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 897366e8..e5b35826 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -104,16 +104,7 @@ module CleanupCallbacks
     original_callbacks = nil
 
     setup do
-      if ActiveRecord::VERSION::MAJOR > 3
-        original_callbacks = target.send(:get_callbacks, type).deep_dup
-      else
-        # While this defeats the purpose of targeted callback
-        # cleanup, callbacks were incredibly difficult to modify
-        # prior to Rails 4, and Rails internal callbacks were only
-        # used for autosaving associations in Rails 3. Our tests
-        # don't care whether a Fluxor's widget is autosaved.
-        target.reset_callbacks(type)
-      end
+      original_callbacks = target.send(:get_callbacks, type).deep_dup
     end
 
     teardown do
diff --git a/test/unit/model_test.rb b/test/unit/model_test.rb
index 9270c130..ad5348dd 100644
--- a/test/unit/model_test.rb
+++ b/test/unit/model_test.rb
@@ -678,39 +678,22 @@ class HasPaperTrailModelTest < ActiveSupport::TestCase
     end
   end
 
-  context "Timestamps" do
-    setup do
-      @wotsit = Wotsit.create! name: "wotsit"
-    end
+  test "update_attributes! records timestamps" do
+    wotsit = Wotsit.create! name: "wotsit"
+    wotsit.update_attributes! name: "changed"
+    reified = wotsit.versions.last.reify
+    assert_not_nil reified.created_at
+    assert_not_nil reified.updated_at
+  end
 
-    should "record timestamps" do
-      @wotsit.update_attributes! name: "changed"
-      assert_not_nil @wotsit.versions.last.reify.created_at
-      assert_not_nil @wotsit.versions.last.reify.updated_at
-    end
-
-    # Tests that it doesn't try to write created_on as an attribute just because
-    # a created_on method exists.
-    #
-    # - Deprecation warning in Rails 3.2
-    # - ActiveModel::MissingAttributeError in Rails 4
-    #
-    # In rails 5, `capture` is deprecated in favor of `capture_io`.
-    #
-    should "not generate warning" do
-      assert_update_raises_nothing = lambda {
-        assert_nothing_raised {
-          @wotsit.update_attributes! name: "changed"
-        }
-      }
-      warnings =
-        if respond_to?(:capture_io)
-          capture_io { assert_update_raises_nothing.call }.last
-        else
-          capture(:stderr) { assert_update_raises_nothing.call }
-        end
-      assert_equal "", warnings
-    end
+  # Tests that it doesn't try to write created_on as an attribute just because
+  # a created_on method exists.
+  #
+  # - ActiveModel::MissingAttributeError in Rails 4
+  #
+  test "update_attributes! does not raise error" do
+    wotsit = Wotsit.create! name: "name1"
+    assert_nothing_raised { wotsit.update_attributes! name: "name2" }
   end
 
   context "A subclass" do
diff --git a/test/unit/protected_attrs_test.rb b/test/unit/protected_attrs_test.rb
deleted file mode 100644
index bac47712..00000000
--- a/test/unit/protected_attrs_test.rb
+++ /dev/null
@@ -1,52 +0,0 @@
-require "test_helper"
-
-class ProtectedAttrsTest < ActiveSupport::TestCase
-  subject { ProtectedWidget.new }
-
-  # These ActiveModel matchers (provided by shoulda-matchers) only work for
-  # Rails 3.
-  if ActiveRecord::VERSION::MAJOR < 4
-    accessible_attrs = ProtectedWidget.accessible_attributes.to_a
-    accessible_attrs.each do |attr_name|
-      should allow_mass_assignment_of(attr_name.to_sym)
-    end
-    inaccessible = ProtectedWidget.
-      column_names.
-      reject { |column_name| accessible_attrs.include?(column_name) }
-    inaccessible.each do |attr_name|
-      should_not allow_mass_assignment_of(attr_name.to_sym)
-    end
-  end
-
-  context "A model with `attr_accessible` created" do
-    setup do
-      @widget = ProtectedWidget.create! name: "Henry"
-      @initial_attributes = @widget.attributes
-    end
-
-    should "be `nil` in its previous version" do
-      assert_nil @widget.paper_trail.previous_version
-    end
-
-    context "which is then updated" do
-      setup do
-        @widget.assign_attributes(name: "Jeff", a_text: "Short statement")
-        @widget.an_integer = 42
-        @widget.save!
-      end
-
-      should "not be `nil` in its previous version" do
-        assert_not_nil @widget.paper_trail.previous_version
-      end
-
-      should "the previous version should contain right attributes" do
-        # For some reason this test seems to be broken in JRuby 1.9 mode in the
-        # test env even though it works in the console. WTF?
-        unless ActiveRecord::VERSION::MAJOR >= 4 && defined?(JRUBY_VERSION)
-          previous_attributes = @widget.paper_trail.previous_version.attributes
-          assert_attributes_equal previous_attributes, @initial_attributes
-        end
-      end
-    end
-  end
-end