This paper reviews the development of X-ray free-electron lasers (XFELs).With the unprecedented characteristics of flexible wavelength tunability,high brightness,ultrashort pulse duration,and fully transverse coherence,these accelerator-based X-ray light sources will open new fields in physics,chemistry,material,and life sciences.Based on the present status of XFELs in the worldwide context,the future directions are analyzed.Abrief summary of the developments of free-electron lasers (FELs) in China is given.