Based on a two-semester course held at the University of Heidelberg, Germany, this book provides a solid basis for postgraduate students wishing to obtain a more profound understanding of the foundations of Quantum Field Theory. The book covers a wide spectrum of topics ranging from traditional operator and modern path integral methods, to different regularization and renormalization methods, asymptotic behavior of Green functions, a particular view on the Renormalization Group, and spontaneous symmetry breaking in effective potentials. Much effort has been made to present the material in a transparent, detailed and structured way, which should help the reader to follow the material.