This review deals with essential physical and biologic aspects of ionizing radiation as related to dermatology. The discussion of major physical factors and basic biophysics includes the production of x rays, dose definitions and recent changes in terminology, various factors affecting dose and tissue damage and current radiotherapy methods. The biologic effects and potential risks of ionizing radiation are reviewed in detail, particularly the major nonstochastic and stochastic somatic effects, such as radiation-induced skin cancer, thyroid carcinoma, breast cancer, and other radiogenic neoplasms. Current methods of risk assessment, radiation carcinogenesis, the efficacy of radiation protection technics, and potential genetic effects are critically evaluated.