Windows is a modern operating system that runs on consumer PCs, laptops, tablets, and phones as well as business desktop PCs, workstations, and enterprise servers. Windows is also the operating system used in Microsoft’s Xbox gaming system, the Hololens mixed-reality device, and Azure cloud computing infrastructure. The most recent version is Windows 11, released in 2021. In this chapter, we will examine various aspects of Windows, starting with a brief history, then moving on to its architecture. After this, we will look at processes, memory management, caching, I/O, the file system, power management, and finally, security.