Search for observational evidence for black holes.

Properties of black holes are discussed which can aid in locating them in the universe. The origin of stellar black holes is recounted, and the problem of black hole invisibility is considered. The most important observational properties are shown to be luminosity due to accretion of interstellar gas and X-rays emitted during accretion in close binary systems. The Uhuru X-ray observations of Cyg X-1 and SMC X-1 are presented as likely evidence of the presence of black holes in those systems. The existence of black holes in globular star clusters is assessed, and it is suggested that galactic nuclei must contain supermassive black holes or some other unusual objects.