This is a reprint of the much-cited 1993 classic, describing the X window system -- the de facto standard windowing system for Linux, UNIX and many other operating systems. Unusually, it covers the whole system so the reader gains a complete understanding of X, and learns how the various components relate to one another -- essential for programming or administering a system. The book has three sections: X in a nutshell -- a quick overview. How X works, in detail, and how the user sees it. Using the system, system administration, performance and programming. While this is a technical book, it doesn't assume advanced programming or sysadmin experience. It is written in a clear, uncomplicated style, with over 200 illustrations. For maximum accessibility, it has a flexible, modular structure that makes it easy for readers to skip to the sections that interest them. The book has been widely recommended as a course text.