Medical physics is a profession specializing in the application of the principles and methodology of the physical sciences to the prevention, diagnosis and treatment of disease. The role of the medical physicist in radiation oncology is diverse and dynamic. The medical physicist interacts with the radiation oncologists, dosimetrists, therapists, patients, nursing staffs, and clinic administrators. The physicist contributes to every aspect of the process that translates into better cancer care. To define more precisely the role physicists play in radiation oncology, this presentation covers three main areas of responsibility: machine quality assurance, patient quality assurance, and radiation safety.