Toxicity of the respiratory system is a common side effect and complication of anticancer therapy that can result in significant morbidity. The range of respiratory compromise can extend from acute lethal events to degrees of chronic pulmonary decompensation, manifesting years after the initial cancer therapy. This review examines the anatomic-histologic background of the lung and the normal functional anatomic unit. The pathophysiology of radiation and chemotherapy induced lung injury is discussed as well as the associated clinical syndromes. Radiation tolerance doses and volumes are assessed in addition to chemotherapy tolerance and risk factors and radiation-chemotherapy interactions. There are a variety of measurable endpoints for detection and screening. Because of the wide range of available quantitative tests, it would seem that the measurement of impaired lung function is possible. The development of staging systems for acute and late toxicity is discussed and a new staging system for Late Effects in Normal Tissues (LENT) is proposed.