This book comprises Robert Geroch's course notes on quantum field theory. Although written in 1971 Geroch's lecture notes are still a very helpful text on quantum field theory since they contain a concise exposition of its core topics accompanied by compressed but deep and clear explanations. What also makes this book a valuable contribution to the existing textbooks on quantum field theory is Geroch's unique approach to teaching theoretical and mathematical physics - the physical concepts and the mathematics, which describes them, are masterfully intertwined in such a way that both reinforce each other to facilitate the understanding of even the most abstract and subtle issues.