Predicted by Einstein's theory of general relativity but so far only indirectly detected, these are the ripples or distortions of space-time produced by the motion of matter. Indirect detection of gravitational waves came from the discovery in 1974 that two pulsars in orbit around each other (PSR 1913 + 16) are gradually getting closer by exactly the amount predicted if the two objects were emitting gravitational waves. Direct detection of gravitational waves may come from future ground and space-based missions now being developed.