Windows Genuine Advantage (WGA) is an anti-piracy system enacted by Microsoft that enforces Microsoft Windows online validation of the authenticity of several recent Microsoft operating systems when accessing several Microsoft Windows services, such as Windows Update, and downloading from the Microsoft Download Center. Specifically, WGA covers Windows XP and Windows Vista. It does not cover Windows 2000, Windows Server 2003 or the Windows 9x family.

It also advertises Service Pack 2 for Windows XP, which requires manual intervention to disable. Previously voluntary, it became mandatory for use of these services in July 2005.