We are now closer than ever to the fundamental goal of physics of understanding all physical phenomena as the inevitable consequences of few simple principles. The grand unified theory of the strong, weak and electromagnetic interactions has, among other things, explained the quantization of charge, determined the magnitude of the neutral current, and had dramatic impact on cosmology. This book is designed to bring interested students and researchers rapidly up to the point where they can contribute to this exciting field. A substantial text provides a review of the subject. In particular, several chapters are devoted to cosmology and the theory of galaxy formation. A selection of original papers is reprinted. A brief review of group theory is also provided. It is a must for all students and researchers in the field.