The key figure behind the development of the theory of relativity is Albert Einstein. Introduced through his famous papers in the early 20th century, particularly the special and general relativity theories, Einstein's work fundamentally altered our understanding of time, space, and gravity. His theories proposed that the laws of physics are invariant across reference frames, and gravity results from the curvature of spacetime caused by mass. Einstein's groundbreaking ideas have had profound implications in both physics and cosmology.