Xlib (also known as libX11) is an X Window System protocol client library written in the C programming language. It contains functions for interacting with an X server. These functions allow programmers to write programs without knowing the details of the X protocol.

Few applications use Xlib directly; rather, they employ other libraries that use Xlib functions to provide widget toolkits:

• X Toolkit Intrinsics (Xt)
• Athena widget set (Xaw)
• Motif
• FLTK
• GTK
• Qt (X11 version)
• Tk
• SDL (Simple DirectMedia Layer)
• SFML (Simple and Fast Multimedia Library)

Xlib appeared around 1985, and is used in GUIs for many Unix-like operating systems. A re-implementation of Xlib was introduced in 2007 using XCB.