Today on stage at the Windows 10 event, Microsoft announced that customers running Windows 7, Windows 8.1, and Windows Phone 8.1 will all be eligible for free upgrades to Windows 10



From Windows Central:

Free upgrades will be available to users running those systems for the first year of Windows 10, and users will get updates for the supported lifetime of their device.

  Windows 10 to be free upgrade for first year for users of Windows 7 and 8.1, Windows Phone 8.1