This book offers an excellent introduction to General Relativity and Cosmology. It is designed to serve as a self-contained text for graduate and advanced undergraduate students and also to provide a basic text for PhD courses. Each of the four parts of the book, two basic and two advanced, can be used as an independent module. In the first part, the main concepts of General Relativity are presented, while the second offers an introduction to the astrophysical applications. The third part is advanced, and discusses the extensions of General Relativity; the contents represent ideal material for a short course at PhD level. The final part of the book provides an introduction to Relativistic Cosmology and its applications. Throughout the text, all mathematical calculations are explained clearly, in step by step detail. Whenever appropriate, the reader is guided to further specialized sources of information.