This book is an introduction to the use of quantum field theory in particle physics intended for both beginning and more advanced theorists, and interested experimenters. Writing in a lucid and engaging style, the authors explain the principal concepts of perturbative field theory and demonstrate their many practical applications. They take great care to make their presentation as self-contained as possible, emphasising a learning-by-doing approach through thorough explanations, numerous examples, and interesting end-of-chapter problems that test the understanding of the subject matter and further amplify the text's main ideas. This book is therefore well suited for self-study and contains enough material for introductory and more advanced courses. It assumes only a basic knowledge of quantum mechanics and special relativity. The content has benefited from being class-tested by the authors over many years and has been used in summer schools worldwide. This title is available as open access on Cambridge Core.