Abstract

This review summarizes the astrophysical evidence for the existence of black holes provided by their gravitational influence on nearby matter. Two classes of accreting black holes have now been observationally verified: supermassive black holes (SMBHs) in galactic nuclei, and stellar-mass black holes in X-ray binaries (XRBs). With the recent re-discovery of ultra-luminous X-ray (ULX) sources, fresh evidence has also emerged for the existence of a third class of accreting black holes: intermediate-mass black holes (IMBHs). The properties of the three classes of accreting black holes are briefly discussed.