Cosmology is a field of astronomy that studies the origin and evolution of the universe. General theory of relativity, also called Einstein's theory of gravity, serves as the foundation for comprehending the history and large-scale structure of the universe. A number of significant predictions about the physical world such as the Big Bang origin of the universe, the existence of black holes, effect of gravity on clocks, and gravitational lensing are also based on general relativity theory. According to this theory, numerous astrophysical occurrences produce gravitational waves, which are ripples in the geometry of spacetime and these propagate at the speed of light. This theory provides a unified description of gravity as a geometric property of space and time, also known as the four-dimensional spacetime model. It also improves upon Newton's law of universal gravitation and generalizes special relativity. It serves as the foundation for the fields of relativistic astrophysics and cosmology. This book unfolds the principles and applications of the general theory relativity. It presents researches and studies performed by experts across the globe. The book will help the readers in keeping pace with the rapid changes in this field.