ActiveSync is a mobile data synchronization technology and protocol developed by Microsoft, originally released in 1996. There are two implementations of the technology: one which synchronizes data and information with handheld devices with a specific desktop computer (originally known as Handheld PC Explorer), and another technology, commonly known as Exchange ActiveSync (or EAS), which provides push synchronization of contacts, calendars, tasks, and email between ActiveSync-enabled servers and devices.

Exchange ActiveSync is a proprietary protocol and is licensed to a number of mobile device companies, including Apple for iOS, Palm for its webOS devices, and Google for certain Android smartphones. Exchange ActiveSync technology is also used by other messaging and collaborative software servers, including Novell GroupWise and Lotus Domino and FuseMetrix. In the Windows Task Manager, the associated process is called wcescomm.exe.

The desktop ActiveSync program allows a mobile device to be synchronized with either a desktop PC, or a server running Microsoft Exchange Server, Atmail, Axigen Mail Server, Horde, PostPath Email and Collaboration Server, Critical Path Messaging Server, Kerio Connect, Scalix, Zimbra or Z-push. Only Personal information manager (PIM) data (Email/Calendar/Contacts) may be synchronized with the Exchange Server. (Tasks may also be synchronized with Exchange Server on Windows Mobile 5.0 devices.) The PC synchronization option, however, allows PIM synchronization with Microsoft Outlook, along with Internet "favorites", files, and tasks, amongst other data types. ActiveSync doesn't support all features of Outlook. For instance, contacts grouped into subfolders aren't transferred. Only the contacts which are not in a subfolder are synchronized.