Abstract: As an electromagnetic wave of short wavelength and high energy, X-rays have been used to probe the fine structure of materials and biological macromolecules for over a century. The first hard X-ray free electron laser (XFEL) facility in the Stanford Linear Accelerator Center national laboratory was commissioned in 2009, and since then its cutting-edge technology has provided highly brilliant, ultrashort pulsed, and fully coherent X-rays, transforming many research fields by expanding the capability of studying fine structures at high temporal and spatial resolutions. The number of XFEL facilities constructed or planned is growing rapidly around the world. In this article, we will explain the principle of an XFEL and summarize the major characteristics of X-rays generated by XFELs. With their unique properties, several milestones have been achieved in the application of XFELs in visualizing the structure and dynamics of molecules in biological sciences.