Radiation delivery to cancer patients for radiotherapy is invariably accompanied by unwanted radiation to other parts of the patient′s body. Traditionally, considerable effort has been made to calculate and measure the radiation dose to the target as well as to nearby critical structures. Only recently has attention been focused also on the relatively low doses that exist far from the primary radiation beams. In several clinical scenarios, such doses have been associated with cardiac toxicity as well as an increased risk of secondary cancer induction. Out-of-field dose is a result of leakage and scatter and generally difficult to predict accurately. The present review aims to present existing data, from measurements and calculations,and discuss its implications for radiotherapy.