ActiveSync is a synchronization program developed by Microsoft. It allows a mobile device to be synchronized with either a desktop PC, or a server running Microsoft Exchange Server. The PC synchronization option, however, allows PIM synchronization with Microsoft Outlook, along with Internet "favorites", files, and tasks, amongst other data types.

Supported mobile devices include Pocket PC's, PDAs or Smartphones running Windows Mobile, or the Windows CE operating system. ActiveSync also provides for the manual transfer of files to a mobile device, along with limited backup/restore functionality, and the ability to install and uninstall mobile device applications.

Starting with Windows Vista, the latest release of the Windows Operating System, ActiveSync has been replaced with the Windows Mobile Device Center.