Unlike most introductory and advanced books on quantum mechanics, this book covers a large number of applications of quantum mechanics including their relativistic generalization and even quantum chemistry. It thereby closes a gap between books of quantum mechanics and quantum field theory. Starting with quantum mechanics, the book then treats the equations of relativistic quantum mechanics with emphasis on Dirac's equation, followed by a discussion of spin to finally arrive at quantum field theory. The relativistic equations are used as a starting point to explain all the small corrections to quantum mechanics leading to measurable effects. The chapters on scattering and bound states convey a deep understanding of the field. Hyperfine shifts and dispersive effects like the Lambshift are treated in a comprehensible manner. The beautiful symmetries on which the particle classifications are based are discussed in subsequent sections. The appendices illustrate technical difficulties encountered in calculations. In the third edition of this successful text, published posthumously, many sections have been revised and widely extended. In particular the discussion of Hyperfine shifts, radiation, dispersive effects, quarks have been thoroughly revisited and split up into three chapters. Sections on Kramer's equation and on electroweak interactions have been added. The book will be of interest to researchers and graduate students in quantum mechanics and quantum field theory.