One of features used in testing of permissions in SharePoint is "Sign in as Different User" which allows you to log in as an another user.  With SharePoint 2013 this option is missing.

To get this feature back follow the following steps :

  1. Locate and then open the following file in a text editor:  C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\TEMPLATE\CONTROLTEMPLATES\Welcome.ascx

  2. Add the following element before the existing "ID_RequestAccess" element:

    <SharePoint:MenuItemTemplate runat="server" ID="ID_LoginAsDifferentUser" Text="<%$Resources:wss,personalactions_loginasdifferentuser%>" Description="<%$Resources:wss,personalactions_loginasdifferentuserdescription%>" MenuGroupId="100" Sequence="100" UseShortId="true" />

  3. Save the file.

Microsoft has published a KB article on the same : http://support.microsoft.com/kb/2752600.

Hope this information was helpful.