Observational evidence suggests that most — if not all — binary X-ray sources are neutron stars. The evolutionary status and possible formation mechanisms of the type I (massive) and type II (low-mass) X-ray binaries are discussed. The difference between the “standard” massive X-ray binaries and the Be/X-ray binaries is ascribed to a somewhat different evolutionary history and status, and possible reasons for the existence of short- and long-period X-ray pulsars are discussed. Type II X-ray sources in globular clusters were most probably formed by capture processes; their formation rate inferred from the observations indicates that only a small fraction (≲ 1 to 10 percent) of the originally formed neutron stars have remained in their clusters. Type II sources in the galactic bulge may also have formed from cataclysmic binaries in which a white dwarf was driven over the Chandrasekhar limit by accretion.