- Published on Wednesday, 29 July 2015 15:40
Windows 10 marks a "new era" for personal computing, Microsoft's chief executive has said.
The software, launched globally on Wednesday, is the company's attempt to reverse its fortunes in the mobile industry.
Windows 10 will be offered as a free upgrade to most consumers.
However, companies will have to pay for their version, as will PC-makers to pre-install it. Analysts say the strategy is designed to speed adoption.
Speaking exclusively to BBC News, Satya Nadella said: "Windows 10 is a huge milestone for us as a company, and quite frankly the industry."
Microsoft is staggering the release over several weeks, so not everyone will be able to get the upgrade on the launch day.
What's in Windows 10 and how do you get it? Read more.
Microsoft has until now released a new version of Windows every few years.
Windows 10 will be the last launch of this kind, the company said - from here on it will gradually update the software for free over months and years.