This book is an examination of the way in which Einstein's general theory of relativity - the theory of gravitation and of space-time, has held up under today's exacting scrutiny of planetary probes, radio astronomy, atomic clocks and electronic super-computers. Clifford M. Will, who has devoted two decades to this question, brings to the general reader a full account of the people, ideas and machines that have put Einstein's theory to the test, covering tests from about 1960 to check the predictions of general relativity accurately and aims to find new predictions to check. He tells the story of relativity from the first faint photographs in 1919 confirming Einstein's prediction of the bending of starlight in the sun's gravitational field, to the design of the delicate gyroscopes now being prepared for a critical space shuttle experiment in the 1990s.