Black Holes are regions of space in which gravitational fields are so strong that no particle or signal can escape the pull of gravity. The boundary of this no-escape region is called the event horizon, since distant observers outside the black hole cannot see (cannot get light from) events inside.Although the fundamental possibility of such an object exists within Newton's classical theory of gravitation, Einstein's theory of gravity makes black holes inevitable under some circumstances. Prior to the early 1960s, black holes seemed to be only an interesting theoretical concept with no astrophysical plausibility, but with the discovery of quasars in 1963 it became clear that very exotic astrophysical objects could exist. Nowadays it is taken for granted that black holes do exist in at least two different forms. Stellar mass black holes are the endpoint of the death of some stars, and supermassive black holes are the result of coalescences in the centers of most galaxies, including our own. No signal can propagate from inside a black hole, but the gravitational influence of a black hole is always present. (This influence does not propagate out of the hole; it is permanently present outside, and depends only on the total amount of mass, angular momentum, and electric charge that have gone into forming the hole.) Black holes can be detected through the influence of this strong gravity on the surroundings just outside the hole. In this way, stellar mass holes produce detectable X-rays, supermassive black holes produce a wide spectrum of electromagnetic signals, and both types can be inferred from the orbital motion of luminous stars and matter around them. Phenomena involving black holes of any mass can produce strong gravitational waves, and are of interest as sources for present and future gravitational wave detectors.