Nick Fox, Google Vice President of Communications Products, writing on the official Google Blog:

Similar to our Nexus hardware program, Project Fi enables us to work in close partnership with leading carriers, hardware makers, and all of you to push the boundaries of what’s possible. By designing across hardware, software and connectivity, we can more fully explore new ways for people to connect and communicate. Two of the top mobile networks in the U.S.—Sprint and T-Mobile—are partnering with us to launch Project Fi and now you can be part of the project too.

Project Fi is an MVNO built on Sprint and T-Mobile’s cellular network which claims to seamlessly switch between the two networks and Wi-Fi depending on whichever will provide the fastest connection in your current location. The service starts at $20 a month for talk, text, tethering, and international coverage. As for data costs, it’s a simple $10 per gigabyte. And if you don’t use all of the data from your chosen plan, you’ll receive credit for your unused data.

At launch, the Nexus 6 is the only device supported by Project Fi and you’ll need to request an invite if you’re interested in using the service.

Project Fi sounds great in concept, but my biggest concern would be what all this network switching is going to do to your device’s battery life. We’ll have to wait and see what reviewers experience when they put it through its paces.