'''APPX''' is the [[file format]] used to distribute and install apps on Windows 8.x, 10, 11, Windows Phone 8.1, Windows 10 Mobile, Xbox One, Xbox seriesSeries S/X|S, Hololens, and Windows 10 IoT Core.<ref>{{cite web|url=http://msdn.microsoft.com/en-us/library/windows/apps/hh464929.aspx |title=App packages and deployment (Windows Runtime apps) |website=[[MSDN|Windows Dev Center]] |publisher=Microsoft |access-date=8 March 2016}}</ref> Unlike legacy desktop apps, APPX is the only installation system allowed for UWP apps. It replaces the [[XAP (file format)|XAP]] file format on [[Windows Phone 8.1]], in an attempt to unify the distribution of apps for Windows Phone and Windows 8.<ref>{{cite web |url=https://www.theverge.com/2014/2/11/5400660/windows-phone-8-1-features-leaked |title=Windows Phone 8.1 includes universal apps and lots of feature updates |first=Tom |last=Warren |date=11 February 2014 |publisher=[[Vox Media]]|work=[[The Verge]]}}</ref> APPX files are only compatible with Windows Phone 8.1 and later versions, and with Windows 8 and later versions.<ref>[http://www.wphoneapps.net/2013/12/how-to-install-xap-file-on-windows-phone.html How to Install APPX File on Windows Phone 8.1] {{webarchive |url=https://web.archive.org/web/20150425033027/http://www.wphoneapps.net/2013/12/how-to-install-xap-file-on-windows-phone.html |date=April 25, 2015 }}</ref>
The Windows Phone 8.x Marketplace allows users to download APPX files to an [[SD Card]] and install them manually. In contrast, sideloading is prohibited on Windows 8.x, unless the user has a developers license or in a business ___domain.<ref>{{cite web|url=http://www.howtogeek.com/129535/how-to-sideload-modern-apps-on-windows-8/ |title=How To Sideload Modern Apps on Windows 8 |work=How-To Geek}}</ref>{{Unreliable source?|date=March 2016}}
