This introductory book, which grew out of lectures given at the Mathematics Institute of Wurzburg University, proposes a combination of coding theory and number theory. Chapter one gives a standard course of linear codes. The next two chapters treat a link between coding theory and number theory. Chapter four is a systematic study of algebraic-geometric codes and in Chapter five a connection between binary linear codes and theta functions is discussed. The book is designed to teach undergraduates and graduates the basic ideas and techniques of coding theory and number theory.