Microsoft Product Activation is a DRM technology used by Microsoft in several of its computer software programs, most notably its Windows operating system and its Office productivity suite. The procedure enforces compliance with the program's end-user license agreement by transmitting information about both the product key used to install the program and the user's computer hardware to Microsoft, inhibiting or completely preventing the use of the program until the validity of its license is confirmed.

The procedure has been met with significant criticism by many consumers, technical analysts and computer experts, who argue that it is poorly designed, highly inconvenient and ultimately does nothing to prevent software piracy. The process has been successfully circumvented on multiple occasions.

This technology is also used in Microsoft Office products during activation. To activate volume-licensed versions of Office, including Project and Visio, one must have a Key Management Service (KMS) host computer. One can configure a Windows Server computer to be a KMS host computer by installing the Volume Activation Services role and then running the Volume Activation Tools wizard.

When installing a retail copy of Windows or Office, the user is asked to input a unique product key supplied on a certificate of authenticity included with the program, which is later verified during activation. Immediate activation is not required following installation, but the program must be activated within a specific period of time in order to continue to function properly. Throughout this grace period, the user will be periodically reminded to activate the program, with warnings becoming more frequent over time.

Certain versions of Windows and Office are available under a volume license, where a single product key is used for multiple installations. Programs purchased under this license must still be activated, with the exception of Windows XP and all versions of Office released prior to Office 2010. Businesses using this licensing system have the option of using Microsoft's activation servers or creating and managing their own.

If Windows is pre-installed on a computer by an original equipment manufacturer (OEM), the operating system is automatically activated without the need for interaction from the user. In this case, the copy of Windows installed does not use the product key listed on the certificate of authenticity, but rather a master product key issued to OEMs called a System Locked Pre-installation (SLP) key. On each boot, Windows confirms the presence of specific information stored in the BIOS by the manufacturer, ensuring the activation only remains valid on that computer, even if the product key is used on another machine.

If activation is not performed within the grace period, or fails because of an illegal or invalid product key, then the following restrictions will be imposed on the user:

the programs will enter a reduced functionality mode, where files can be viewed but not edited.