The Windows Insider Program is a way for enthusiasts to gain early access to the latest changes to Microsoft Windows, and a way for Microsoft to test their software for bugs across a much wider range of devices than they could gather on their own, and also a way to gauge interest in potential new features.

Microsoft has offered “beta testing” programs for products like Windows and Office for years, back to the days of special Usenet channels, but the Windows Insider program started in 2014 with the upcoming launch of Windows 10. Initially expecting no more than 40,000 testers, the Windows Insider Program quickly grew to millions of testers, reaching some 18 million at one point.

Potential testers can learn more about the Insider Program from Microsoft at the Windows Insider site, and register to try out the latest versions of Windows.

Microsoft initially broke Insider builds into “Rings,” with a Fast Ring for the most cutting edge builds, the Slow Ring for more stable and closer to shipping releases, and the Release Preview Ring for versions of Windows just about to ship. This later changed into a series of Channels, to align better with Microsoft’s other Insider programs for Office, etc. Currently, there are four Channels: the Canary Channel, a potentially unstable release containing features that may or may not make it into shipping versions of Windows, the Dev Channel, for again long range feature testing, but not quite as cutting edge as Canary, the Beta Channel, and the Release Preview Channel.