The data from animals, cell lines, and humans have led to the consensus of induction of carcinogenesis by ionizing radiation, especially at low-level doses, and that there is a dose–response relationship between radiation and cancer incidence. However, additional factors, including radiation type, dose rate, specific tissues, and animal species, also provide a contribution. The development of molecular biology research has helped explain the mechanism of radiation carcinogenesis, including pathway activation and chromosome alterations. Bystander effects and abscopal effects are additionally characteristics of radiation carcinogenesis. This chapter takes a look at how radiation, from both environment and industry, has contributed to cancer incidence over the past century.