This is a draft version of Part I of a three-part introductory textbook
on quantum field theory. The book is based on a one-year course that
have taught off-and-on for the past 20 years. My goal is to present the basic
concepts and formalism of QFT as simply and straightforwardly as possible,
emphasizing its logical structure. To this end, I have tried to take the simplest
possible example of each phenomenon, and then to work through it in great
detail.