The WatchOS is the operating system that is found on the Apple Watch device. This has been developed by Apple Inc. and it has many similarities to the iOS operating system. It was originally released in April 2015 at the same time the original Apple Watch was introduced to the public. The Apple Watch is the only device that runs the WatchOS. The updated version, the WatchOS 4 was released on September 19, 2017 and this book will consider the different things that are available on the latest operating system for the Apple Watch.