This course is part of?The Applied Sciences of Oncology (ASO) distance-learning course.Aim:?To provide an understanding of the radiobiological principles that affect the results of clinical radiotherapy.ObjectivesUpon completion, participants should be able to:Describe clinical relevance of radiobiology.Discuss acute and late effects of ionising radiation in a range of organs and tissue typesDiscuss late effects of radiotherapyDiscuss the effect of time and fraction size on tumor and normal tissueDiscuss radiobiological aspects of brachytherapyDiscuss?biological effects of radiotherapy at a cellular and molecular levelDiscuss?interactions between radiotherapy and chemotherapy at a molecular level.