Abstract: X-ray free electron lasers (FELs) are large-scale scientific facilities that hold great promise for the generation of coherent short-wavelength radiation with ultra-high brightness and ultra-short pulse duration. They enable unprecedented exploration of physics, chemistry, material science, biology, and so forth, with spatial and temporal resolution down to the atomic and molecular level. This article gives an overview of the basic principle, historical development, current operation modes, and state-of-the-art of X-ray FEL facilities throughout the world, as well as the rapid development in recent years of X-ray FELs in China.