Stellar black holes in close binary systems are observable when matter is transferred from a companion star to the black hole; the energy release in the fall toward the compact star is so large that the matter heats up to temperatures of several hundred million degrees and radiates in X-rays. The black hole therefore is observable in X-rays, whereas the companion star can be observed with optical telescopes. The energy release for black holes and neutron stars are of the same order of magnitude. Black holes and neutron stars are therefore often difficult to distinguish.