The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) established by the General Assembly in 1955 assesses the levels and effects of exposure to ionizing radiation on human health and the environment. This is the first of two volumes of scientific annexes that summarizes the current state of knowledge regarding the risks of second primary cancer following radiotherapy and future research needs. It is important for establishing the basis for long-term medical care and preventive interventions that will benefit the growing population of cancer survivors.