Institute for Advanced Study,
School of Natural Sciences Princeton,
New Jersey 08540, USA
Black holes are very puzzling objects that are predicted by general relativity. We review some of their interesting classical properties. Once we combine general relativity and quantum mechanics, black holes cease to be completely black. In fact, they emit thermal radiation. The existence of this thermal radiation poses paradoxes. Using string theory, as a theory of quantum gravity, one can resolve some of these paradoxes. This leads to some interesting changes in our conceptual understanding of space and time.