This book is an introduction to -algebras and their representations on Hilbert spaces. The presentation is as simple and concrete as possible; the book is written for a second-year graduate student who is familiar with the basic results of functional analysis, measure theory and Hilbert spaces. The author does not aim for great generality, but confines himself to the best-known and also to the most important parts of the theory and the applications. Because of the manner in which it is written, the book should be of special interest to physicists for whom it opens an important area of modern mathematics. In particular, chapter 1 can be used as a bare-bones introduction to -algebras where sections 2.1 and 2.3 contain the basic structure theory for Type 1 von Neumann algebras.