Twentieth- the century began with two illustrious theories in Physics - Relativity and Quantum Mechanics, which completely changed our view about nature and force. While Quantum Mechanics deals with micro-particles, Relativity deals with particles moving at very high speed (comparable to the speed of light). Although Schrodinger's equation works well with micro-particles, it cannot handle particles at high speed. In truth, all the microparticles we deal with possess high speed. Hence, we have to look for an equation that can cope with micro- particles of high speed. Such an equation was provided by P.A.M Dirac. Apart from handling relativistic microparticles, the Dirac equation predicts the spin of the particles and also explains the negative energy state. Any relativistic phenomenon requires the understanding of Tensors. The first few chapters of the book, therefore, deals with the understanding of Tensors. The book is written in a self-study mode. Each step is clear and problems are solved to explain the concept better. The book, though an introductory one, can ignite a positive passion for the subject.