Professor Kapusta develops the basic formalism and theoretical techniques for studying relativistic quantum field theory at high temperature and density. Topics covered include functional integral representation of the partition function, diagrammatic expansions, linear response theory, screening and collective oscillations, equations of state, phase transitions, restoration of spontaneously broken symmetries, the Goldstone theorem, and infrared problems. Specific physical theories treated include QED, QCD, the Weinberg-Salam model, and effective nuclear field theories. Applications to white dwarfs, neutron stars, ultrarelativistic nucleus-nucleus collisions, and the early universe are discussed. Problems are provided at the end of each chapter, and numerous references to the literature are included.