diff --git a/activestorage/lib/active_storage/service.rb b/activestorage/lib/active_storage/service.rb
index c8f675db86..f2e1269f27 100644
--- a/activestorage/lib/active_storage/service.rb
+++ b/activestorage/lib/active_storage/service.rb
@@ -97,7 +97,7 @@ module ActiveStorage
# Returns a signed, temporary URL that a direct upload file can be PUT to on the +key+.
# The URL will be valid for the amount of seconds specified in +expires_in+.
- # You most also provide the +content_type+, +content_length+, and +checksum+ of the file
+ # You must also provide the +content_type+, +content_length+, and +checksum+ of the file
# that will be uploaded. All these attributes will be validated by the service upon upload.
def url_for_direct_upload(key, expires_in:, content_type:, content_length:, checksum:)
raise NotImplementedError
diff --git a/guides/source/active_storage_overview.md b/guides/source/active_storage_overview.md
new file mode 100644
index 0000000000..2ab14ad3de
--- /dev/null
+++ b/guides/source/active_storage_overview.md
@@ -0,0 +1,549 @@
+**DO NOT READ THIS FILE ON GITHUB, GUIDES ARE PUBLISHED ON http://guides.rubyonrails.org.**
+
+Active Storage
+==============
+
+This guide covers how to attach files to your Active Record models.
+
+After reading this guide, you will know:
+
+* How to attach one or many files to a record.
+* How to delete an attached file.
+* How to link to an attached file.
+* How to use variants to transform images.
+* How to generate an image representation of a non-image file, such as a PDF or a video.
+* How to send file uploads directly from browsers to a storage service,
+ bypassing your application servers.
+* How to clean up files stored during testing.
+* How to implement support for additional storage services.
+
+--------------------------------------------------------------------------------
+
+What is Active Storage?
+-----------------------
+
+Active Storage facilitates uploading files to a cloud storage service like
+Amazon S3, Google Cloud Storage, or Microsoft Azure Storage and attaching those
+files to Active Record objects. It comes with a local disk-based service for
+development and testing and supports mirroring files to subordinate services for
+backups and migrations.
+
+Using Active Storage, an application can transform image uploads with
+[ImageMagick](https://www.imagemagick.org), generate image representations of
+non-image uploads like PDFs and videos, and extract metadata from arbitrary
+files.
+
+## Setup
+
+Active Storage uses two tables in your application’s database named
+`active_storage_blobs` and `active_storage_attachments`. After upgrading your
+application to Rails 5.2, run `rails active_storage:install` to generate a
+migration that creates these tables. Use `rails db:migrate` to run the
+migration.
+
+You need not run `rails active_storage:install` in a new Rails 5.2 application:
+the migration is generated automatically.
+
+Declare Active Storage services in `config/storage.yml`. For each service your
+application uses, provide a name and the requisite configuration. The example
+below declares three services named `local`, `test`, and `s3`:
+
+```yaml
+local:
+ service: Disk
+ root: <%= Rails.root.join("storage") %>
+
+test:
+ service: Disk
+ root: <%= Rails.root.join("tmp/storage") %>
+
+s3:
+ service: S3
+ access_key_id: ""
+ secret_access_key: ""
+```
+
+Tell Active Storage which service to use by setting
+`Rails.application.config.active_storage.service`. Because each environment will
+likely use a different service, it is recommended to do this on a
+per-environment basis. To use the disk service from the previous example in the
+development environment, you would add the following to
+config/environments/development.rb:
+
+```ruby
+# Store files locally.
+config.active_storage.service = :local
+```
+
+To use the s3 service in production, you add the following to
+`config/environments/production.rb`:
+
+```ruby
+# Store files in S3.
+config.active_storage.service = :s3
+```
+
+Continue reading for more information on the built-in service adapters (e.g.
+`Disk` and `S3`) and the configuration they require.
+
+### Disk Service
+
+Declare a Disk service in `config/storage.yml`:
+
+``` yaml
+local:
+ service: Disk
+ root: <%= Rails.root.join("storage") %>
+```
+
+### Amazon S3 Service
+
+Declare an S3 service in `config/storage.yml`:
+
+``` yaml
+s3:
+ service: S3
+ access_key_id: ""
+ secret_access_key: ""
+ region: ""
+ bucket: ""
+```
+Also, add the S3 client gem to your Gemfile:
+
+``` ruby
+gem "aws-sdk-s3", require: false
+```
+
+### Microsoft Azure Storage Service
+
+Declare an Azure Storage service in `config/storage.yml`:
+
+``` yaml
+azure:
+ service: AzureStorage
+ path: ""
+ storage_account_name: ""
+ storage_access_key: ""
+ container: ""
+```
+
+Also, add the Microsoft Azure Storage client gem to your Gemfile:
+
+``` ruby
+gem "azure-storage", require: false
+```
+
+### Google Cloud Storage Service
+
+Declare a Google Cloud Storage service in `config/storage.yml`:
+
+``` yaml
+google:
+ service: GCS
+ keyfile: {
+ type: "service_account",
+ project_id: "",
+ private_key_id: "",
+ private_key: "",
+ client_email: "",
+ client_id: "",
+ auth_uri: "https://accounts.google.com/o/oauth2/auth",
+ token_uri: "https://accounts.google.com/o/oauth2/token",
+ auth_provider_x509_cert_url: "https://www.googleapis.com/oauth2/v1/certs",
+ client_x509_cert_url: ""
+ }
+ project: ""
+ bucket: ""
+```
+
+Also, add the Google Cloud Storage client gem to your Gemfile:
+
+``` ruby
+gem "google-cloud-storage", "~> 1.3", require: false
+```
+
+### Mirror Service
+
+You can keep multiple services in sync by defining a mirror service. When a file
+is uploaded or deleted, it's done across all the mirrored services. Mirrored
+services can be used to facilitate a migration between services in production.
+You can start mirroring to the new service, copy existing files from the old
+service to the new, then go all-in on the new service. Define each of the
+services you'd like to use as described above and reference them from a mirrored
+service.
+
+``` yaml
+s3_west_coast:
+ service: S3
+ access_key_id: ""
+ secret_access_key: ""
+ region: ""
+ bucket: ""
+
+s3_east_coast:
+ service: S3
+ access_key_id: ""
+ secret_access_key: ""
+ region: ""
+ bucket: ""
+
+production:
+ service: Mirror
+ primary: s3_east_coast
+ mirrors:
+ - s3_west_coast
+```
+
+NOTE: Files are served from the primary service.
+
+Attach Files to a Model
+-----------------------
+
+### `has_one_attached`
+
+The `has_one_attached` macro sets up a one-to-one mapping between records and
+files. Each record can have one file attached to it.
+
+For example, suppose your application has a User model. If you want each user to
+have an avatar, define the `User` model like this:
+
+``` ruby
+class User < ApplicationRecord
+ has_one_attached :avatar
+end
+```
+
+You can create a user with an avatar:
+
+``` ruby
+class SignupController < ApplicationController
+ def create
+ user = Users.create!(user_params)
+ session[:user_id] = user.id
+ redirect_to root_path
+ end
+
+ private
+ def user_params
+ params.require(:user).permit(:email_address, :password, :avatar)
+ end
+end
+```
+
+Call `avatar.attach` to attach an avatar to an existing user:
+
+```ruby
+Current.user.avatar.attach(params[:avatar])
+```
+
+Call `avatar.attached?` to determine whether a particular user has an avatar:
+
+```ruby
+Current.user.avatar.attached?
+```
+
+### `has_many_attached`
+
+The `has_many_attached` macro sets up a one-to-many relationship between records
+and files. Each record can have many files attached to it.
+
+For example, suppose your application has a `Message` model. If you want each
+message to have many images, define the Message model like this:
+
+```ruby
+class Message < ApplicationRecord
+ has_many_attached :images
+end
+```
+
+You can create a message with images:
+
+```ruby
+class MessagesController < ApplicationController
+ def create
+ message = Message.create!(message_params)
+ redirect_to message
+ end
+
+ private
+ def message_params
+ params.require(:message).permit(:title, :content, images: [])
+ end
+end
+```
+
+Call `images.attach` to add new images to an existing message:
+
+```ruby
+@message.images.attach(params[:images])
+```
+
+Call `images.attached?` to determine whether a particular message has any images:
+
+```ruby
+@message.images.attached?
+```
+
+Remove File Attached to Model
+-----------------------------
+
+To remove an attachment from a model, call `purge` on the attachment. Removal
+can be done in the background if your application is setup to use Active Job.
+Purging deletes the blob and the file from the storage service.
+
+```ruby
+# Synchronously destroy the avatar and actual resource files.
+user.avatar.purge
+
+# Destroy the associated models and actual resource files async, via Active Job.
+user.avatar.purge_later
+```
+
+Link to Attachments
+-------------------
+
+Generate a permanent URL for the blob that points to the application. Upon
+access, a redirect to the actual service endpoint is returned. This indirection
+decouples the public URL from the actual one, and allows, for example, mirroring
+attachments in different services for high-availability. The redirection has an
+HTTP expiration of 5 min.
+
+```ruby
+url_for(user.avatar)
+```
+
+To create a download link, use the `rails_blob_{path|url}` helper. Using this
+helper allows you to set the disposition.
+
+```ruby
+rails_blob_path(user.avatar, disposition: "attachment")
+```
+
+Transform Images
+----------------
+
+To create variation of the image, call `variant` on the Blob.
+You can pass any [MiniMagick](https://github.com/minimagick/minimagick)
+supported transformation to the method.
+
+To enable variants, add `mini_magick` to your Gemfile:
+
+``` ruby
+gem 'mini_magick'
+```
+
+When the browser hits the variant URL, ActiveStorage will lazy transform the
+original blob into the format you specified and redirect to its new service
+location.
+
+```erb
+<%= image_tag user.avatar.variant(resize: "100x100") %>
+```
+
+Preview Non-image Files
+-----------------------
+
+Some non-image files can be previewed: that is, they can be presented as images.
+For example, a video file can be previewed by extracting its first frame. Out of
+the box, Active Storage supports previewing videos and PDF documents.
+
+```erb
+
+ <% @message.files.each do |file| %>
+ -
+ <%= image_tag file.preview(resize: "100x100>") %>
+
+ <% end %>
+
+```
+
+WARNING: Extracting previews requires third-party applications, `ffmpeg` for
+video and `mutool` for PDFs. These libraries are not provided by Rails. You must
+install them yourself to use the built-in previewers. Before you install and use
+third-party software, make sure you understand the licensing implications of
+doing so.
+
+Upload Directly to Service
+--------------------------
+
+Active Storage, with its included JavaScript library, supports uploading
+directly from the client to the cloud.
+
+### Direct upload installation
+
+1. Include `activestorage.js` in your application's JavaScript bundle.
+
+ Using the asset pipeline:
+
+ ```js
+ //= require activestorage
+
+ ```
+
+ Using the npm package:
+
+ ```js
+ import * as ActiveStorage from "activestorage"
+ ActiveStorage.start()
+ ```
+
+2. Annotate file inputs with the direct upload URL.
+
+ ```ruby
+ <%= form.file_field :attachments, multiple: true, direct_upload: true %>
+ ```
+3. That's it! Uploads begin upon form submission.
+
+### Direct upload JavaScript events
+
+| Event name | Event target | Event data (`event.detail`) | Description |
+| --- | --- | --- | --- |
+| `direct-uploads:start` | `