Abstract: This paper reviews Einstein’s great contribution in establishing the theory of relativity, especially general relativity. Einstein proposed the principle of the constancy of the speed of light, the principle of general covariance, Mach’s principle, and the equivalence principle.He was the first person to point out that gravitation is essentially a geometrical effect of the curvature of space-time, and also to give the fundamental equation of general relativity. Why Einstein is recognized as the sole founder of both special relativity and general relativity is also discussed.