A black hole is a point of extreme mass in space-time with a radius, or event horizon, inside of which all electromagnetic radiation (including light) is trapped by gravity. A black hole is an extremely compact object, collapsed by gravity which has overcome electric and nuclear forces. It is believed that stars appreciably larger than the Sun, once they have exhausted all their nuclear fuel, collapse to form black holes: they are "black" because no light escapes their intense gravity. Material attracted to a black hole, though, gains enormous energy and can radiate part of it before being swallowed up. Some astronomers believe that enormously massive black holes exist in the centre of our galaxy and of other galaxies. This book brings together leading research from throughout the world.