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