The nuclear power plant accident at Chornobyl released large quantities of Iodine-I31 into the atmosphere, contaminating thousands of square kilometers and exposing millions of people. For this study, a well-defined cohort of approximately 12,000 Ukrainian children aged 0-17 at the time of the accident has been examined for thyroid disease by well-trained specialists every two years for a four screening cycles and the fifth cycle screening is underway. The study is a collaborative effort of researchers in Ukraine and the United States. Under a standardized protocol subjects have received thyroid examinations, including palpation, ultrasound scanning, and thyroid hormone and other laboratory tests. Information relevant for dose reconstruction is collected at interview. The study has expanded to include record linkage with the National Cancer Registry and molecular genetic and genomic investigations of radiation-related thyroid cancer.