The book is based on courses given by E. Hewitt at the University of Washington and the University of Uppsala. The book is intended to be readable by students who have had basic graduate courses in real analysis, set-theoretic topology, and algebra. That is, the reader should know elementary set theory, set-theoretic topology, measure theory, and algebra. The book begins with preliminaries in notation and terminology, group theory, and topology. It continues with elements of the theory of topological groups, the integration on locally compact spaces, and invariant functionals. The book concludes with convolutions and group representations, and characters and duality of locally compact Abelian groups.