Graduate students in many branches of mathematics need to know something about topological groups and the Haar integral to enable them to understand applications in their own fields. In this introduction to the subject, Professor Higgins covers the basic theorems they are likely to need, assuming only some elementary group theory. The book is based on lecture courses given for the London M.Sc. degree in 1969 and 1972, and the treatment is more algebraic than usual, reflecting the interests of the author and his audience. The volume ends with an informal account of one important application of the Haar integral, to the representation theory of compact groups, and suggests further reading on this and similar topics.