Heavy electrons are found among a number of lanthanide and actinide compounds, and are characterized by a large effective mass which becomes comparable to the mass of a muon. Heavy electrons exhibit rich phenomena such as unconventional superconductivity, weak anti- ferromagnetism, or pseudo meta-magnetism. This book is intended not only as a monograph, but can readily serve as an advanced textbook on theoretical and experimental physics of strongly correlated electrons. Over the last two decades, heavy electrons have been the focus of very active experimental and theoretical studies. Many established ideas and techniques have been insufficient to describe and understand heavy electrons and their impact properly. On the theoretical side, quantum fluctuations make mean-field theories difficult to handle, while on the experimental side, extreme conditions such as strong magnetic fields and pressure at ultra-low temperatures may be required. Heavy electron systems as described in this book offer a case study for applying and testing most of the major tools in theoretical and experimental condensed matter physics. Graduate students and researchers working on strongly correlated condensed matter systems will find in this book a comprehensive introduction and many examples how conventional concepts of solids may work or not work, and how they can be refined and sharpened in the context of heavy electron systems.