In this course, the existence of maximal finite invariant speed (speed of light) leads to the many interesting consequences. The classical Newtonian theory breaks at high speed. The classical concepts of space and time breaks and the new concept of spacetime arises in which space and time are in the same footing. And our usual intuition breaks at this speeds. We observe lots of new phenomena which are relativistic in nature. Some examples are theory of Electrodynamics, Relativistic Quantum Mechanics, Quantum Field theory. Here, the theory of simultaneity is explained exclusively. Also, all the concepts are discussed very intuitively using very simple and effective examples. The importance of different frame of references have been discussed. Also, the introductory concept on gauge theory has been discussed. All the derivations are carry out using unique method – Principle of least Action. By completing this course, one will get experienced in the Special theory of relativity. By using the concepts and derivations taught in this lectures series, one can easily understand the advanced theoretical physics course like Quantum electrodynamics, Quantum field theory, Standard model etc. Also, the students will get some exposer to the General Theory of relativity which is the geometric description of the Gravity.