Defining organs at risk is a crucial task for radiation oncologists when aiming to optimize the benefit of radiation therapy, with delivery of the maximum dose to the tumor volume while sparing healthy tissues. This book will prove an invaluable guide to the delineation of organs at risk of toxicity in patients undergoing radiotherapy. The first and second sections address the anatomy of organs at risk, discuss the pathophysiology of radiation-induced damage, and present dose constraints and methods for target volume delineation. The third section is devoted to the radiological anatomy of organs at risk as seen on typical radiotherapy planning CT scans, with a view to assisting the radiation oncologist to recognize and delineate these organs for each anatomical region - head and neck, mediastinum, abdomen, and pelvis. The book is intended both for young radiation oncologists still in training and for their senior colleagues wishing to reduce intra-institutional variations in practice and thereby to standardize the definition of clinical target volumes.