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. Contents: IntroductionReview of the Standard 123 TheoryGrand UnificationSO(10)Exceptional UnificationReality and Complexity of the WorldProton DecayFamily Problem and Orthogonal UnificationFermion Mass HierarchyA Short Course in CosmologyGenesis of MatterIntroduction to the Theory of Galaxy FormationNeutrinos and GalaxiesMonopoles and InflationHierarchy TechnicolorSupersymmetry, and VariationsInvisible AxionsComposite Quarks and LeptonsGravity and Grand UnificationAppendix A: Instant Review of Group TheoryAppendix B: Two-Component FormalismSelected Original Papers Readership: Graduate students and researchers interested in grand unifiedtheory.