It's important to note that it's not a single theory, but rather two distinct theories:

* Special Relativity: Published in 1905, this theory deals with the relationship between space and time, and how they are affected by motion.

* General Relativity: Published in 1915, this theory extends special relativity to include gravity, explaining it as a curvature of spacetime caused by mass and energy.

While Einstein is credited with the discovery and development of these theories, his work built upon the ideas and contributions of other physicists like Hendrik Lorentz, Henri Poincaré, and Hermann Minkowski.