The 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. 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. It pays special attention to applications that go beyond graphical user interfaces (GUIs), such as visualization and imaging programs; issues affecting video games; 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.