Quantum field theory is a fundamental branch of theoretical physics, which in the last 20 years, has led to spectacular progress in our understanding of phase transitions and elementary particle physics. This textbook emphasizes the underlying unity of the concepts and methods used in both domains, and presents in clear language topics such as the pertubative expansion, Feynman diagrams, renormalization, and the renormalization group. It contains detailed applications of critical phenomena to condensed matter physics, such as the calculation of critical exponents and a discussion of the XY model. Applications to particle physics include quantum electrodynamics and chromodynamics, electroweak interactions, and lattice gauge theories. The book is based on courses given over several years on statistical mechanics and field theory, and is written at graduate level. It attempts to guide the reader through a somewhat difficult and sometimes intricate subject. Further it is intended to lead the student up to the level of more advanced textbooks and research articles. A large number of problems are given to measure the reader's understanding.