An understanding of the properties and interactions of the elementary particles is an essential prerequisite of research work in high energy physics. Much progress in the subject has been achieved with the aid of symmetry principles. In this 1980 book the concept of symmetry or invariance is employed as a unifying theme. Using a careful explanation of the mathematical formalism and with many applications to particular cases, the authors introduce the reader to the symmetry schemes which dominate the world of the particle physicist. The presentation will also appeal to mathematicians and physicists in other fields who are interested in the applications of the general principles of symmetry. After a brief survey of the particles and a review of the relevant quantum mechanics, the principal symmetries are studied in turn. Some technical points are relegated to appendices and the book contains extensive references.