In [[computing]], '''Windows on Windows''' (commonly referred to as '''WOW'''),<ref>{{cite web|url=http://support.microsoft.com/kb/181333|title=WOW Environment Remains in Memory After Quitting 16-Bit Program|website=Support|publisher=[[Microsoft]]|access-date=February 7, 2017|date=February 22, 2007|url-status=dead|archive-url=https://web.archive.org/web/20071023060218/http://support.microsoft.com/kb/181333|archive-date=October 23, 2007}}</ref><ref>{{cite web|url=http://support.microsoft.com/kb/153544|title=Starting 16-Bit WOW Subsystem on Windows NT Server|date=November 1, 2016|access-date=February 7, 2017|website=Support|publisher=[[Microsoft]]|url-status=dead|archive-url=https://web.archive.org/web/20070509051612/http://support.microsoft.com/kb/153544|archive-date=May 9, 2007}}</ref><ref>{{cite web|url=https://support.microsoft.com/kb/220159|title=Disabling the MSDOS and WOWEXEC Subsystems on Terminal Server|date=November 1, 2006|website=Support|publisher=[[Microsoft]]|access-date=February 7, 2017|url-status=live|archive-url=https://web.archive.org/web/20080113000651/http://support.microsoft.com/kb/220159|archive-date=January 13, 2008}}</ref> was a [[compatibility layer]] of [[32-bit]] versions of the [[Windows NT]] family of [[operating system]]s since 1993 with the release of [[Windows NT 3.1]], which extends [[Virtual DOS machine#Windows NTVDM|NTVDM]] to provide limited support for running [[legacy code|legacy]] [[16-bit]] programs written for [[Windows 3.x]] or earlier. There is a similar subsystem, known as [[WoW64]], on 64-bit Windows versions that runs 32-bit programs.
This subsystem is not available in 64-bit editions since [[Windows 11]] (including [[Windows Server 2008 R2]] and later, which only have 64-bit editions) and therefore cannot run 16-bit software without third-party emulation software (e.g. [[DOSBox]]). [[Windows 10]] is the final version of Windows to include this subsystem.
