X-ray astronomy is the prime available window on astrophysical compact objects: black holes, neutron stars and white dwarfs. New observational opportunities have led to an explosion of knowledge in this field. This book provides a comprehensive overview of the astrophysics of compact objects that emit X-rays. Sixteen chapters written by the foremost experts in the field cover the observations and the astrophysical interpretation of these objects. Topics covered include binary systems, gamma ray burst sources, anomalous X-ray pulsars, super-soft sources, and enigmatic fast X-ray transients. Further chapters are dedicated to isolated neutron stars and the X-ray source populations of globular clusters. The properties of X-ray binaries are discussed in depth in chapters on quasi-periodic oscillations and related aperiodic X-ray variability, X-ray bursts, black holes, and relativistic jets. This is a valuable reference for both graduate students and active researchers.