This textbook presents a careful introduction to group theory and its applications in atomic, molecular and solid-state physics. The reader is provided with the necessary background on the mathematical theory of groups and then shown how group theory is a powerful tool for solving physics problems. Worked examples and exercises with hints and answers encourage self-study, while the inclusion of some advanced subjects, such as the theory of induced representations and ray representations, Racah theory of atomic spectra, and Landau theory of second-order phase transitions, should interest professionals.