The visible universe has edges, known as event horizons, which surround a black hole or a region of space speeding away faster than light. Event horizons are governed by a strikingly simple set of quantum laws which imply that black holes are at once the simplest and most complex objects in the physical universe. As discovered four decades ago by Bekenstein and Hawking, this dichotomy underlies a deep paradox, the resolution of which has become a focal point of modern physics. Unexpectedly, insight into this paradox has recently been gained from string theory. In this lecture, Andrew Strominger, will give an elementary account of these developments.