Windows Phone 8.1, rolled out at Microsoft's Build Conference in San Francisco, California, on April 2, 2014, is the third generation of Microsoft's Windows Phone mobile operating system, succeeding Windows Phone 8. It was released in final form to Windows Phone developers on April 14, 2014 and reached general availability on July 15, 2014. All Windows Phones running Windows Phone 8 will be upgraded to Windows Phone 8.1, with release dependent on carrier rollout dates.