Radiotherapy remains the major non-surgical treatment modality for malignant disease, with over 50% of patients receiving radiotherapy at some point in their treatment. The physics behind the administration of radiotherapy is essential to an understanding of how the treatment works, and therefore ensuring optimum practice. However, most oncologists will have been taught little physics since school. This resource is specifically for the oncology and radiation team. It begins with basic concepts and then explores the principles and practice of physics as it relates to radiotherapy, including discussion of specific types of therapy. It is mapped against the FRCR curriculum to serve as essential reading for the trainee oncologist and the wider radiotherapy team.