In [[computing]], '''Windows on Windows''' or '''Windows on Win32'''<ref>{{Cite web |title=W |url=http://msdn.microsoft.com/en-us/library/windows/hardware/ff556347(v=vs.85).aspx |archive-url=http://web.archive.org/web/20121103104537/http://msdn.microsoft.com/en-us/library/windows/hardware/ff556347(v=vs.85).aspx |archive-date=2012-11-03 |access-date=2025-07-23 |website=msdn.microsoft.com}}</ref> (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> is a discontinued [[compatibility layer]] of [[32-bit computing|32-bit]] versions of the [[Windows NT]] family of [[operating system]]s. Since 1993, with the release of [[Windows NT 3.1]], WoW extends [[Virtual DOS machine#Windows NTVDM|NTVDM]] to provide limited support for running [[legacy system|legacy]] [[16-bit computing|16-bit]] programs written for [[Windows 3.x]] or earlier. There is a similar subsystem, known as [[WoW64]], on [[64-bit computing|64-bit]] Windows versions that runs 32-bit programs.
This subsystem will be retired with the [[Product lifecycle#Phase 4: Service|end of support]] of [[Windows 10]] in October 14, 2025. The last version of Windows to include this subsystem is Windows 10, as [[Windows 11]] (and [[Windows Server 2008 R2]] and later) only run the [[x86-64]] processor in [[long mode]] and therefore cannot run 16-bit software without emulation software.
