This textbook is intended to be used in an introductory course in quantum field theory. It assumes the standard undergraduate education of a physics major and it is designed to appeal to a wide array of physics graduate students, from those studying theoretical and experimental high energy physics to those interested in condensed matter, optical, atomic, nuclear and astrophysicists. It includes a thorough development of the field theoretic approach to nonrelativistic many-body physics as a step in developing a broad-based working knowledge of some of the basic aspects of quantum field theory. It presents a logical, step by step systematic development of relativistic field theory and of functional techniques and their applications to perturbation theory with Feynman diagrams, renormalization, and basic computations in quantum electrodynamics.