diff --git a/docs/guide/README.md b/docs/guide/README.md
index 7de23d71f0..10b36e02ca 100644
--- a/docs/guide/README.md
+++ b/docs/guide/README.md
@@ -111,7 +111,7 @@ Security
 * [Authentication](security-authentication.md)
 * [Authorization](security-authorization.md)
 * [Working with Passwords](security-passwords.md)
-* **TBD** [Auth Clients](security-auth-clients.md)
+* [Auth Clients](security-auth-clients.md)
 * [Best Practices](security-best-practices.md)
 
 
diff --git a/docs/guide/security-auth-clients.md b/docs/guide/security-auth-clients.md
index 688fa89708..f23c31c8c3 100644
--- a/docs/guide/security-auth-clients.md
+++ b/docs/guide/security-auth-clients.md
@@ -1,6 +1,383 @@
 Auth Clients
 ============
 
-> Note: This section is under development.
->
-> It has no content yet.
+Yii provides official extension that lets you authenticate and/or authorize using external services via consuming
+[OpenID](http://openid.net/), [OAuth](http://oauth.net/) or [OAuth2](http://oauth.net/2/).
+
+Installing extension
+--------------------
+
+In order to install extension use Composer. Either run
+                                            
+```
+composer require --prefer-dist yiisoft/yii2-authclient "*"
+```
+
+or add
+
+```json
+"yiisoft/yii2-authclient": "*"
+```
+to the `require` section of your composer.json.
+
+Configuring clients
+-------------------
+
+After extension is installed you need to setup auth client collection application component:
+
+```php
+'components' => [
+    'authClientCollection' => [
+        'class' => 'yii\authclient\Collection',
+        'clients' => [
+            'google' => [
+                'class' => 'yii\authclient\clients\GoogleOpenId'
+            ],
+            'facebook' => [
+                'class' => 'yii\authclient\clients\Facebook',
+                'clientId' => 'facebook_client_id',
+                'clientSecret' => 'facebook_client_secret',
+            ],
+            // etc.
+        ],
+    ]
+    ...
+]
+```
+
+Out of the box the following clients are provided:
+
+- [[\yii\authclient\clients\Facebook|Facebook]].
+- [[yii\authclient\clients\GitHub|GitHub]].
+- Google (via [[yii\authclient\clients\GoogleOpenId|OpenID]] and [[yii\authclient\clients\GoogleOAuth|OAuth]]).
+- [[yii\authclient\clients\LinkedIn|LinkedIn]].
+- [[yii\authclient\clients\Live|Microsoft Live]].
+- [[yii\authclient\clients\Twitter|Twitter]].
+- [[yii\authclient\clients\VKontakte|VKontakte]].
+- Yandex (via [[yii\authclient\clients\YandexOpenId|OpenID]] and [[yii\authclient\clients\YandexOAuth|OAuth]]).
+
+Configuration for each client is a bit different. For OAuth it's required to get client ID and secret key from
+the service you're going to use. For OpenID it works out of the box in most cases.
+
+Storing authorization data
+--------------------------
+
+In order to recognize the user authenticated via external service we need to store ID provided on first authentication
+and then check against it on subsequent authentications. It's not a good idea to limit login options to external
+services only since these may fail and there won't be a way for the user to log in. Instead it's better to provide
+both external authentication and good old login and password.
+
+If we're storing user information in a database the schema could be the following:
+
+```sql
+CREATE TABLE user (
+    id int(11) NOT NULL AUTO_INCREMENT PRIMARY KEY,
+    username varchar(255) NOT NULL,
+    auth_key varchar(32) NOT NULL,
+    password_hash varchar(255) NOT NULL,
+    password_reset_token varchar(255),
+    email varchar(255) NOT NULL,
+    status smallint(6) NOT NULL DEFAULT 10,
+    created_at int(11) NOT NULL,
+    updated_at int(11) NOT NULL
+);
+
+CREATE TABLE auth (
+    id int(11) NOT NULL AUTO_INCREMENT PRIMARY KEY,
+    user_id int(11) NOT NULL,
+    source string(255) NOT NULL,
+    source_id string(255) NOT NULL
+);
+
+ALTER TABLE auth ADD CONSTRAINT fk-auth-user_id-user-id
+FOREIGN KEY user_id REFERENCES auth(id);
+```
+
+In the SQL above `user` is a standard table that is used in advanced application template to store user
+info. Each user can authenticate using multiple external services therefore each `user` record can relate to
+multiple `auth` records. In the `auth` table `client_name` is the name of the auth provider used and `client_id` is
+unique user identificator that is provided by external service after successful login.
+
+Using tables created above we can generate `Auth` model. No further adjustments needed.
+
+
+Adding action to controller
+---------------------------
+
+Next step is to add [[yii\authclient\AuthAction]] to a web controller. Typically `SiteController`:
+
+```php
+class SiteController extends Controller
+{
+    public function actions()
+    {
+        return [
+            'auth' => [
+                'class' => 'yii\authclient\AuthAction',
+                'successCallback' => [$this, 'onAuthSuccess'],
+            ],
+        ];
+    }
+
+    public function onAuthSuccess($client)
+    {
+       $attributes = $client->getUserAttributes();
+
+        /** @var Auth $auth */
+        $auth = Auth::find()->where([
+            'source' => $client->getId(),
+            'source_id' => $attributes['id'],
+        ])->one();
+        
+        if (Yii::$app->user->isGuest) {
+            if ($auth) { // login
+                $user = $auth->user;
+                Yii::$app->user->login($user);
+            } else { // signup
+                if (User::find()->where(['email' => $attributes['email']])->exists()) {
+                    Yii::$app->getSession()->setFlash('error', [
+                        Yii::t('app', "User with the same email as in {client} account already exists but isn't linked to it. Login using email first to link it.", ['client' => $client->getTitle()]),
+                    ]);
+                } else {
+                    $password = Yii::$app->security->generateRandomString(6);
+                    $user = new User([
+                        'username' => $attributes['login'],
+                        'email' => $attributes['email'],
+                        'password' => $password,
+                    ]);
+                    $user->generateAuthKey();
+                    $user->generatePasswordResetToken();
+                    $transaction = $user->getDb()->beginTransaction();
+                    if ($user->save()) {
+                        $auth = new Auth([
+                            'user_id' => $user->id,
+                            'source' => $client->getId(),
+                            'source_id' => (string)$attributes['id'],
+                        ]);
+                        if ($auth->save()) {
+                            $transaction->commit();
+                            Yii::$app->user->login($user);
+                        } else {
+                            print_r($auth->getErrors());
+                        }
+                    } else {
+                        print_r($user->getErrors());
+                    }
+                }
+            }
+        } else { // user already logged in
+            if (!$auth) { // add auth provider
+                $auth = new Auth([
+                    'user_id' => Yii::$app->user->id,
+                    'source' => $client->getId(),
+                    'source_id' => $attributes['id'],
+                ]);
+                $auth->save();
+            }
+        }
+    }
+}
+```
+
+`successCallback` method is called when user was successfully authenticated via external service. Via `$client` instance
+we can retrieve information received. In our case we'd like to:
+ 
+- If user is guest and record found in auth then log this user in.
+- If user is logged in and record found in auth then try connecting additional account (save its data into auth table).
+- If user is guest and record not found in auth then create new user and make a record in auth table. Then log in.
+
+Although, all clients are different they shares same basic interface [[yii\authclient\ClientInterface]],
+which governs common API.
+
+Each client has some descriptive data, which can be used for different purposes:
+
+- `id` - unique client id, which separates it from other clients, it could be used in URLs, logs etc.
+- `name` - external auth provider name, which this client is match too. Different auth clients
+  can share the same name, if they refer to the same external auth provider.
+  For example: clients for Google OpenID and Google OAuth have same name "google".
+  This attribute can be used inside the database, CSS styles and so on.
+- `title` - user friendly name for the external auth provider, it is used to present auth client
+  at the view layer.
+
+Each auth client has different auth flow, but all of them supports `getUserAttributes()` method,
+which can be invoked if authentication was successful.
+
+This method allows you to get information about external user account, such as ID, email address,
+full name, preferred language etc.
+
+Defining list of attributes, which external auth provider should return, depends on client type:
+
+- [[yii\authclient\OpenId]]: combination of `requiredAttributes` and `optionalAttributes`.
+- [[yii\authclient\OAuth1]] and [[yii\authclient\OAuth2]]: field `scope`, note that different
+  providers use different formats for the scope.
+  
+### Getting additional data via extra API calls
+
+Both [[yii\authclient\OAuth1]] and [[yii\authclient\OAuth2]] provide method `api()`, which
+can be used to access external auth provider REST API. However this method is very basic and
+it may be not enough to access full external API functionality. This method is mainly used to
+fetch the external user account data.
+
+To use API calls, you need to setup [[yii\authclient\BaseOAuth::apiBaseUrl]] according to the
+API specification. Then you can call [[yii\authclient\BaseOAuth::api()]] method:
+
+```php
+use yii\authclient\OAuth2;
+
+$client = new OAuth2;
+
+// ...
+
+$client->apiBaseUrl = 'https://www.googleapis.com/oauth2/v1';
+$userInfo = $client->api('userinfo', 'GET');
+```
+
+Adding widget to login view
+---------------------------
+
+There's ready to use [[yii\authclient\widgets\AuthChoice]] widget to use in views:
+
+```php
+<?= yii\authclient\widgets\AuthChoice::widget([
+     'baseAuthUrl' => ['site/auth'],
+     'popupMode' => false,
+]) ?>
+```
+
+Creating your own auth clients
+------------------------------
+
+You may create your own auth client for any external auth provider, which supports
+OpenId or OAuth protocol. To do so, first of all, you need to find out which protocol is
+supported by the external auth provider, this will give you the name of the base class
+for your extension:
+
+ - For OAuth 2 use [[yii\authclient\OAuth2]].
+ - For OAuth 1/1.0a use [[yii\authclient\OAuth1]].
+ - For OpenID use [[yii\authclient\OpenId]].
+
+At this stage you can determine auth client default name, title and view options, declaring
+corresponding methods:
+
+```php
+use yii\authclient\OAuth2;
+
+class MyAuthClient extends OAuth2
+{
+    protected function defaultName()
+    {
+        return 'my_auth_client';
+    }
+
+    protected function defaultTitle()
+    {
+        return 'My Auth Client';
+    }
+
+    protected function defaultViewOptions()
+    {
+        return [
+            'popupWidth' => 800,
+            'popupHeight' => 500,
+        ];
+    }
+}
+```
+
+Depending on actual base class, you will need to redeclare different fields and methods.
+
+### [[yii\authclient\OpenId]]
+
+All you need is to specify auth URL, by redeclaring `authUrl` field.
+You may also setup default required and/or optional attributes.
+For example:
+
+```php
+use yii\authclient\OpenId;
+
+class MyAuthClient extends OpenId
+{
+    public $authUrl = 'https://www.my.com/openid/';
+
+    public $requiredAttributes = [
+        'contact/email',
+    ];
+
+    public $optionalAttributes = [
+        'namePerson/first',
+        'namePerson/last',
+    ];
+}
+```
+
+### [[yii\authclient\OAuth2]]
+
+You will need to specify:
+
+- Auth URL by redeclaring `authUrl` field.
+- Token request URL by redeclaring `tokenUrl` field.
+- API base URL by redeclaring `apiBaseUrl` field.
+- User attribute fetching strategy by redeclaring `initUserAttributes()` method.
+
+For example:
+
+```php
+use yii\authclient\OAuth2;
+
+class MyAuthClient extends OAuth2
+{
+    public $authUrl = 'https://www.my.com/oauth2/auth';
+
+    public $tokenUrl = 'https://www.my.com/oauth2/token';
+
+    public $apiBaseUrl = 'https://www.my.com/apis/oauth2/v1';
+
+    protected function initUserAttributes()
+    {
+        return $this->api('userinfo', 'GET');
+    }
+}
+```
+
+You may also specify default auth scopes.
+
+> Note: Some OAuth providers may not follow OAuth standards clearly, introducing
+  differences, and may require additional efforts to implement clients for.
+
+### [[yii\authclient\OAuth1]]
+
+You will need to specify:
+
+- Auth URL by redeclaring `authUrl` field.
+- Request token URL by redeclaring `requestTokenUrl` field.
+- Access token URL by redeclaring `accessTokenUrl` field.
+- API base URL by redeclaring `apiBaseUrl` field.
+- User attribute fetching strategy by redeclaring `initUserAttributes()` method.
+
+For example:
+
+```php
+use yii\authclient\OAuth1;
+
+class MyAuthClient extends OAuth1
+{
+    public $authUrl = 'https://www.my.com/oauth/auth';
+
+    public $requestTokenUrl = 'https://www.my.com/oauth/request_token';
+
+    public $accessTokenUrl = 'https://www.my.com/oauth/access_token';
+
+    public $apiBaseUrl = 'https://www.my.com/apis/oauth/v1';
+
+    protected function initUserAttributes()
+    {
+        return $this->api('userinfo', 'GET');
+    }
+}
+```
+
+You may also specify default auth scopes.
+
+> Note: Some OAuth providers may not follow OAuth standards clearly, introducing
+  differences, and may require additional efforts to implement clients for.
+
diff --git a/extensions/authclient/README.md b/extensions/authclient/README.md
index 946ca472fc..ca22d292cb 100644
--- a/extensions/authclient/README.md
+++ b/extensions/authclient/README.md
@@ -1,7 +1,8 @@
 AuthClient Extension for Yii 2
 ==============================
 
-This extension adds [OpenID](http://openid.net/), [OAuth](http://oauth.net/) and [OAuth2](http://oauth.net/2/) consumers for the Yii framework 2.0.
+This extension adds [OpenID](http://openid.net/), [OAuth](http://oauth.net/) and [OAuth2](http://oauth.net/2/) consumers
+for the Yii framework 2.0.
 
 
 Installation
@@ -12,7 +13,7 @@ The preferred way to install this extension is through [composer](http://getcomp
 Either run
 
 ```
-php composer.phar require --prefer-dist yiisoft/yii2-authclient "*"
+composer require --prefer-dist yiisoft/yii2-authclient "*"
 ```
 
 or add
@@ -51,7 +52,7 @@ You need to setup auth client collection application component:
 ]
 ```
 
-Then you need to add [[yii\authclient\AuthAction]] to some of your web controllers:
+Then you need to add [[yii\authclient\AuthAction]] to web controller:
 
 ```php
 class SiteController extends Controller
@@ -100,33 +101,33 @@ Although, all clients are different they shares same basic interface [[yii\authc
 which governs some common API.
 
 Each client has some descriptive data, which can be used for different purposes:
- - id - unique client id, which separates it from other clients, it could be used in URLs, logs etc.
- - name - external auth provider name, which this client is match too. Different auth clients
+ - `id` - unique client id, which separates it from other clients, it could be used in URLs, logs etc.
+ - `name` - external auth provider name, which this client is match too. Different auth clients
  can share the same name, if they refer to the same external auth provider.
  For example: clients for Google OpenID and Google OAuth have same name "google".
  This attribute can be used inside the database, CSS styles and so on.
- - title - user friendly name for the external auth provider, it is used to present auth client
+ - `title` - user friendly name for the external auth provider, it is used to present auth client
  at the view layer.
 
-Each auth client has different auth flow, but all of them supports "getUserAttributes()" method,
+Each auth client has different auth flow, but all of them supports `getUserAttributes()` method,
 which can be invoked if authentication was successful.
 This method allows you to get information about external user account, such as id, email address,
 full name, preferred language etc.
 Defining list of attributes, which external auth provider should return, depends on client type:
- - [[yii\authclient\OpenId]]: combination of "requiredAttributes" and "optionalAttributes"
- - [[yii\authclient\OAuth1]] and [[yii\authclient\OAuth2]]: field "scope", note that different
+ - [[yii\authclient\OpenId]]: combination of `requiredAttributes` and `optionalAttributes`
+ - [[yii\authclient\OAuth1]] and [[yii\authclient\OAuth2]]: field `scope`, note that different
  providers use different formats for the scope.
 
-Each auth client has "viewOptions" attribute. It is an array, which stores name-value pairs,
+Each auth client has `viewOptions` attribute. It is an array, which stores name-value pairs,
 which serve to compose client representation in the view.
-For example widget [[yii\authclient\widgets\AuthChoice]] uses keys "popupWidth" and "popupHeight" to
+For example widget [[yii\authclient\widgets\AuthChoice]] uses keys `popupWidth` and `popupHeight` to
 determine the size of authentication popup window.
 
 
 External API usage
 ------------------
 
-Both [[yii\authclient\OAuth1]] and [[yii\authclient\OAuth2]] provide method "api()", which
+Both [[yii\authclient\OAuth1]] and [[yii\authclient\OAuth2]] provide method `api()`, which
 can be used to access external auth provider REST API. However this method is very basic and
 it may be not enough to access full external API functionality. This method is mainly used to
 fetch the external user account data.
@@ -147,20 +148,20 @@ Predefined auth clients
 -----------------------
 
 This extension provides the list of ready to use auth clients, which covers most
-popular external authentication providers. These clients are located under "yii\authclient\clients"
+popular external authentication providers. These clients are located under `yii\authclient\clients`
 namespace.
 
 Following predefined auth clients are available:
- - [[yii\authclient\clients\Facebook]] - [Facebook](https://www.facebook.com/) OAuth2 client
- - [[yii\authclient\clients\GitHub]] - [GitHub](https://github.com/) OAuth2 client
- - [[yii\authclient\clients\GoogleOAuth]] - [Google](https://www.google.com/) OAuth2 client
- - [[yii\authclient\clients\GoogleOpenId]] - [Google](https://www.google.com/) OpenID client
- - [[yii\authclient\clients\LinkedIn]] - [LinkedIn](http://www.linkedin.com/) OAuth2 client
- - [[yii\authclient\clients\Live]] - [Microsoft Live](http://live.com/) OAuth2 client
- - [[yii\authclient\clients\Twitter]] - [Twitter](https://twitter.com/) OAuth1 client
- - [[yii\authclient\clients\VKontakte]] - [VKontakte](http://vk.com/) OAuth2 client
- - [[yii\authclient\clients\YandexOAuth]] - [Yandex](http://www.yandex.ru/) OAuth2 client
- - [[yii\authclient\clients\YandexOpenId]] - [Yandex](http://www.yandex.ru/) OpenID client
+ - [[yii\authclient\clients\Facebook]] - [Facebook](https://www.facebook.com/) OAuth2 client.
+ - [[yii\authclient\clients\GitHub]] - [GitHub](https://github.com/) OAuth2 client.
+ - [[yii\authclient\clients\GoogleOAuth]] - [Google](https://www.google.com/) OAuth2 client.
+ - [[yii\authclient\clients\GoogleOpenId]] - [Google](https://www.google.com/) OpenID client.
+ - [[yii\authclient\clients\LinkedIn]] - [LinkedIn](http://www.linkedin.com/) OAuth2 client.
+ - [[yii\authclient\clients\Live]] - [Microsoft Live](http://live.com/) OAuth2 client.
+ - [[yii\authclient\clients\Twitter]] - [Twitter](https://twitter.com/) OAuth1 client.
+ - [[yii\authclient\clients\VKontakte]] - [VKontakte](http://vk.com/) OAuth2 client.
+ - [[yii\authclient\clients\YandexOAuth]] - [Yandex](http://www.yandex.ru/) OAuth2 client.
+ - [[yii\authclient\clients\YandexOpenId]] - [Yandex](http://www.yandex.ru/) OpenID client.
 
 Please, refer to the particular client class documentation for its actual usage.
 
@@ -199,9 +200,10 @@ You may create your own auth client for any external auth provider, which suppor
 OpenId or OAuth protocol. To do so, first of all, you need to find out which protocol is
 supported by the external auth provider, this will give you the name of the base class
 for your extension:
- - for OAuth 2 use [[yii\authclient\OAuth2]]
- - for OAuth 1/1.0a use [[yii\authclient\OAuth1]]
- - for OpenID use [[yii\authclient\OpenId]]
+
+ - For OAuth 2 use [[yii\authclient\OAuth2]].
+ - For OAuth 1/1.0a use [[yii\authclient\OAuth1]].
+ - For OpenID use [[yii\authclient\OpenId]].
 
 At this stage you can determine auth client default name, title and view options, declaring
 corresponding methods:
@@ -233,9 +235,9 @@ class MyAuthClient extends OAuth2
 
 Depending on actual base class, you will need to redeclare different fields and methods.
 
-1) [[yii\authclient\OpenId]]
+### [[yii\authclient\OpenId]]
 
-All you need is specify auth URL, by redeclaring "authUrl" field.
+All you need is to specify auth URL, by redeclaring `authUrl` field.
 You may also setup default required and/or optional attributes.
 For example:
 
@@ -257,13 +259,14 @@ class MyAuthClient extends OpenId
 }
 ```
 
-2) [[yii\authclient\OAuth2]]
+### [[yii\authclient\OAuth2]]
 
 You will need to specify:
-- authorize URL - redeclare "authUrl" field
-- token request URL - redeclare "tokenUrl" field
-- API base URL - redeclare "apiBaseUrl" field
-- User attribute fetching strategy - redeclare "initUserAttributes" method
+
+- Auth URL by redeclaring `authUrl` field.
+- Token request URL by redeclaring `tokenUrl` field.
+- API base URL by redeclaring `apiBaseUrl` field.
+- User attribute fetching strategy by redeclaring `initUserAttributes()` method.
 
 For example:
 
@@ -287,17 +290,18 @@ class MyAuthClient extends OAuth2
 
 You may also specify default auth scopes.
 
-Note: some OAuth providers may not follow OAuth standards clearly, introducing
-some differences, which may require additional efforts to apply.
+> Note: Some OAuth providers may not follow OAuth standards clearly, introducing
+  differences, and may require additional efforts to implement clients for.
 
-3) [[yii\authclient\OAuth1]]
+### [[yii\authclient\OAuth1]]
 
 You will need to specify:
-- authorize URL - redeclare "authUrl" field
-- request token URL - redeclare "requestTokenUrl" field
-- access token URL - redeclare "accessTokenUrl" field
-- API base URL - redeclare "apiBaseUrl" field
-- User attribute fetching strategy - redeclare "initUserAttributes" method
+
+- Auth URL by redeclaring `authUrl` field.
+- Request token URL by redeclaring `requestTokenUrl` field.
+- Access token URL by redeclaring `accessTokenUrl` field.
+- API base URL by redeclaring `apiBaseUrl` field.
+- User attribute fetching strategy by redeclaring `initUserAttributes()` method.
 
 For example:
 
@@ -323,5 +327,6 @@ class MyAuthClient extends OAuth1
 
 You may also specify default auth scopes.
 
-Note: some OAuth providers may not follow OAuth standards clearly, introducing
-some differences, which may require additional efforts to apply.
+> Note: Some OAuth providers may not follow OAuth standards clearly, introducing
+  differences, and may require additional efforts to implement clients for.
+