Radiotherapy plays an important role in the treatment of head and neck cancer. The effects of radiotherapy on the oral hard and soft tissues may range from a mild mucositis to severe caries and osteoradionecrosis. It is the responsibility of the dentist to treat and prevent the severe sequelae which may arise. This paper outlines the effects of radiotherapy on the oral structures and discusses the dental treatment of the patient during radiotherapy. Emphasis is placed on the prevention of complications. The head and neck cancer patient must be thoroughly examined by the dentist and treatment planning may then be divided into pre-irradiaton treatment, treatment during irradiation and post-irradiation treatment. The dentist thus forms an essential member of the team treating patients with head and neck cancer.