This textbook, based on courses taught at Harvard University, is an introduction to group theory and its application to physics. The physical applications are considered as the mathematical theory is developed so that the presentation is unusually cohesive and well-motivated. Many modern topics are dealt with, and there is much discussion of the group SU(n) and its representations. This is of great significance in elementary particle physics. Applications to solid state physics are also considered. This stimulating account will prove to be an essential resource for senior undergraduate students and their teachers.