Quantum field theory has become the framework for the discussion of all fundamental interactions except gravity, and for the understanding of second order phase transitions in statistical mechanics. This advanced text for theoretical particle physicists and statistical physicists has been updated to reflect the improved presentation that has resulted from the author's experience in teaching graduate courses. It approaches the subject in terms of path and functional integrals, adopting a Euclidian metric and using the language of partition and correlation functions. Newly added topics include holomorphic or coherent state path integral and non-relativistic limit of massive quantum fields, and each chapter concludes with exercises and solutions.