Relativity and Practical Applications is a comprehensive textbook designed for undergraduate and postgraduate students of physics and is also suitable for pre-doctoral coursework across universities, colleges, and research institutions. The book presents the foundations, mathematical formulation, and modern applications of relativity in a clear and pedagogical manner, combining physical intuition with rigorous derivations.The text begins with the historical and conceptual development of relativity, including discussions on space, time, ether, light propagation, Galilean relativity, and key experiments such as the Michelson–Morley experiment. It then develops the Special Theory of Relativity through Lorentz transformations, simultaneity, time dilation, length contraction, spacetime geometry, four-vectors, relativistic kinematics and dynamics, and electrodynamics.Advanced chapters introduce relativistic hydrodynamics, relativistic Lagrangian formulation, equivalence principle, gravitation, tensor methods, rotation and Lorentz groups, relativistic kinetic theory, and relativistic thermodynamics. The book also discusses classic paradoxes of relativity, including the twin paradox and Ehrenfest paradox, helping students develop deeper conceptual understanding.A distinctive feature of the book is its emphasis on practical and modern applications relevant to particle physics, accelerator physics, astrophysics, cosmology, and relativistic fluid dynamics. Numerous worked examples, exercises, solved problems, illustrative diagrams, and application-oriented discussions are included throughout the chapters to strengthen conceptual understanding and analytical problem-solving skills. The carefully designed problems and detailed solutions make the book particularly useful for classroom teaching, self-study, competitive examinations, and advanced university coursework.