For 200 years, black holes have been a solution looking for a problem. In the last two decades they have moved to become a real scientific phenomenon. Black holes exist both as stars in our own Galaxy (identifiable as members of binary stars that emit X-rays) and as the centres of energetic galaxies (‘active galactic nuclei’). The last decade has seen the first accurate mass measurements of these black holes, which lie in the range from five times the mass of our own Sun to many million times. We also have clear evidence for the way that black holes accrete material as their primary energy source. Furthermore, the presence of super-massive black holes has been established in some nearby galaxies, and in the centre of our own Galaxy, from their gravitational effect on nearby stars. Some observations show processes that occur very close to the black hole, where the field of gravity is strong enough to test General Relativity. Recent observations suggest the existence of intermediate mass black holes of an as yet unknown origin.