The theory of relativity was developed by Albert Einstein. He introduced two theories: the special theory of relativity in 1905 and the general theory of relativity in 1915. Special relativity revolutionized our understanding of space and time, establishing that the laws of physics are the same for all observers and that the speed of light is constant in a vacuum. General relativity expanded on this by describing gravity not as a force but as the curvature of spacetime caused by mass. Einstein's theories have had profound implications in physics, influencing everything from cosmology to modern technology.