This book explains in detail and without digressions both special and general relativity. The first section deals with the fundamentals of special relativity and provides explanations for the famous equivalence between mass and energy. It also explains why Einstein was able to use the theory of electrodynamics as a basis for his famous paper "On the Electrodynamics of Moving Bodies," as both the speed of light and electric charge are absolute, leading to the relativity of other physical quantities. Following this, the reader is introduced to general relativity. The special relativity theory and the equivalence principle are referenced, and it is explained how spacetime is curved under gravity. The climax of the book consists of Einstein's gravitational equations.