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 antiferromagnetism and pseudo metamagnetism, however, many of the established ideas and techniques relating to theoretical and experimental physics of strongly correlated electrons are insufficient to understand heavy electrons. On the theoretical side, enormous amounts of quantum fluctuations have brought disaster to celebrated mean-field theories. On the other hand, extreme experimental conditions are required such as applying strong magnetic fields and pressure at ultralow temperatures. This book is a case study for applying and testing almost all tools in theoretical and experimental condensed-matter physics to heavy electron systems. Graduate students and researchers who want to work on strongly correlated condensed-matter systems will find in the book many examples of how the conventional concepts on solids work or do not work in heavy -electron systems.