Observational properties of extragalactic X-ray sources.

The current status of observations of extragalactic X-ray sources is reviewed. Attention is given to the Uhuru and Ariel 5 sky surveys, identifications of X-ray clusters of galaxies, cluster morphology, the X-ray luminosity function and spectra of clusters, and the structure of nearby rich clusters, particularly the Perseus, Virgo, and Coma clusters. X-ray observations of Seyfert galaxies are discussed, with emphasis on the X-ray Seyferts as a class and the variability of Seyfert X radiation. Results are also summarized for X-ray observations of radio galaxies, QSOs, BL Lacterids, and high-excitation narrow-emission-line galaxies.