This book provides a comprehensive introduction to the fundamental principles and concepts of Classical, Newtonian and Relativistic Mechanics. It seeks to explain the basic concepts of these areas of physics in a concise manner. The book is primarily meant to serve the requirements of the first-year undergraduate (B.Sc.) students of physics.The initial chapters of the book cover topics such as vector calculus, coordinate systems, conservation laws and frames of reference. The discussion then moves on to inverse square law of forces, particle collisions, and dynamics of rigid bodies. Universal Frame of Reference and Special Theory of Relativity are the major topics covered in the later chapters. Along with the original concept, the modified concept of Michelson-Morley experiment has been presented with its mathematical treatment. The book contains numerous illustrations and many solved examples that help the student in understanding the relevant concepts clearly. A large number of chapter-end questions of descriptive, short answer, and numerical varieties will help to test the students' grasp of the topics covered. In this third edition, the treatment of several concepts covered in the earlier edition has been expanded, and new examples, exercises and several multiple choice questions have been added.