Windows Phone 7 Series, originally codenamed "Photon", is a version of the Windows Phone operating system developed by Microsoft. Windows Phone 7 Series was initially intended to be released during 2009, but several delays prompted Microsoft to develop Windows Mobile 6.5 as an interim release.

The Windows Phone 7 Series features a new operating system and integration with Xbox Live and Zune services. The interface, known as "Metro", has been completely revised and is visually similar to the interface of the Zune HD. All Windows Phone 7 Series devices feature three buttons (Back button, Start button and Search button) and FM Radio capabilities. Microsoft has redesigned the home screen, using "tiles" which scroll vertically and can be customized as quick launches, links to contacts, or self-contained widgets. Windows Phone 7 Series devices have more friendly touchscreen UI with multi-touch technology. A Zune-style user experience, Xbox Live and social networking apps are integrated.