This introduction to the theory of quantum fields in curved spacetime, intended for mathematicians, arose from a course taught to graduate students and is designed for self-study or advanced courses in relativity and quantum field theory. The style is informal and some knowledge of general relativity and differential geometry is assumed, yet the author does supply background material on function analysis and quantum field theory as required. Physicists should also gain a sound grasp of various aspects of the theory, some of which have not been particularly emphasized in the existing review literature.