Thermonuclear reactions in stars is a major topic in the field of nuclear astrophysics, and deals with the topics of how precisely stars generate their energy through nuclear reactions, and how these nuclear reactions create the elements the stars, planets and - ultimately - we humans consist of. The present book treats these topics in detail. It also presents the nuclear reaction and structure theory, thermonuclear reaction rate formalism and stellar nucleosythesis. The topics are discussed in a coherent way, enabling the reader to grasp their interconnections intuitively. The book serves both as a textbook, with many examples and end-of-chapter exercises, but also as a reference book for use by researchers working in the field of nuclear astrophysics.