Windows 10 will be a free upgrade for all customers currently using Windows 8.1 and Windows Phone 8.1
The new operating system, which marks a major update for Microsoft's multi-platform software, will also be a free upgrade for Windows 7 during its first year.
This is a departure from Microsoft's usual policy of charging for its major updates, but now it's eager to get as many users upgraded as possible, and has adopted the free model also used by Apple and its Mac OS X software.
Windows 10 runs on computers, laptops, tablets and smartphones, as well as two-in-one devices.
Microsoft is showing off the latest build of Windows 10 - plus the promise of new Cortana features and hardware - at a media event right now. Head over to our live blog for all of the latest news.