This book provides an overview of the X Window System focusing on characteristics that have significant impact on the development of both application programs and widgets. We pay special attention to applications that go beyond graphical user interfaces (GUIs); therefore we discuss issues affecting video games, visualization and imaging programs, and designing widgets with a complex appearance. While the book does not assume previous knowledge of X, it is intended for experienced programmers, especially those who want to write programs that go beyond simple GUIs. X is the dominant window system under Unix, and X servers are available for Microsoft Windows, thus enabling graphics over a network in the PC world. While Java offers an apparently universal graphics library (the abstract window toolkit), the reality is quite different: For high-quality graphics and image display, we must program on the target platform itself (X or one of Microsoft’s APIs) rather than rely on Java peer objects. X is a vast subject, so it is impossible to provide a complete coverage in a few hundred pages. Thus we selected topics that are fundamental to the system, so that the reader who masters them should be able to read the documentation of the numerous libraries and toolkits. Therefore we provide documentation on the most important Xlib and X toolkit functions only.