The Childhood Cancer Survivor Study (CCSS) will investigate the long-term effects of cancer and its associated therapies. A retrospective cohort study will be conducted through a multi-institutional collaboration, which will involve the identification and active follow-up of a cohort of approximately 50,000 survivors of cancer, diagnosed before 21 years of age, between 1970 and 1999 and 10,000 sibling controls. This project will study children and young adults exposed to specific therapeutic modalities, including radiation, chemotherapy, and/or surgery, who are at increased risk of late-occurring adverse health outcomes. A group of sibling controls will be identified and data collected for comparison purposes.