“Theory of Relativity” explores Albert Einstein's groundbreaking theories, which revolutionized our understanding of physics, space, and time. The book delves into both special and general relativity, explaining how these concepts reshaped our comprehension of gravity and the cosmos. It highlights how, contrary to previous beliefs, space and time are not absolute but are intertwined as a single continuum called spacetime. A key insight is Einstein's famous equation, E=mc², demonstrating the equivalence of mass and energy. The book progresses logically, beginning with the historical context that led to Einstein's work. It then introduces the postulates of special relativity, followed by an exploration of general relativity, including the principle of equivalence and the curvature of spacetime. Experimental evidence supporting these theories, such as gravitational lensing, is examined. Making complex ideas approachable, the book uses clear explanations and analogies. This approach ensures that readers gain a solid grasp of relativity's profound implications for cosmology, astrophysics, and our ongoing exploration of the universe.