The main aim of this book is to give a self-contained and representative cross section through present-day research in solid-state physics. This covers metallic and mesoscopic transport, localization by disorder and superconductivity, including questions related to high-temperature superconductors and to heavy fermion systems. An important part of the book is devoted to itinerant-electron magnetism, discussing paramagnons, strong correlation, magnetization fluctuations and spin density waves. All the formal tools used in these chapters are developed in the first part of the book which contains a thorough discussion of second quantization, and of perturbation theory for an arbitrary complex time path, and also describes the functional approach to Feynman diagrams, including general ward identities. Each chapter contains an extensive list of the relevant literature and a series of problems with detailed solutions which complement the main text. The book is meant as both a course and a research tool.