This book takes the reader from the preliminary ideas of the Special Theory of Relativity (STR) to the doorsteps of the General Theory of Relativity (GTR). The first part explains the main concepts in a layman's language, including STR, the Lorentz transformation, and relativistic mechanics. After that the concept of tensors is built up in detail, especially Maxwell's stress tensor with illustrative examples, culminating in the energy-momentum conservation in electromagnetic fields. The mathematical structure of Minkowski's space-time is constructed and explained graphically. The equation of motion is formulated and then illustrated by the example of the relativistic rocket. The principle of covariance is presented with the covariant equations of classical electrodynamics. Finally, the book constructs the energy tensor, which constitutes the source term in Einstein's field equation, clears the passage to the GTR. In the book, the concepts of tensors are developed carefully and many numerical examples are taken from atomic and nuclear physics. The graphs of essential equations are included. This is suitable for classical electrodynamics, modern physics, and relativity studies.