Radiation is energy that travels in the form of waves or high-speed particles through space or through a material medium. Ionizing radiation and nonionizing radiations are two types of radiation. Ionizing radiation is a double-edged sword. It is used in medical practice in the treatment of cancer, in diagnostic imaging, and in therapeutic or diagnostic radioisotopes. Nonionizing UV is used for therapy in skin diseases and laser therapy for diabetic retinopathy. Whole-body irradiation, stochastic effects, and teratogenic effects are various effects of radiation. Biologic effects mainly depend on rate of delivery, field size, cell proliferation, oxygen effects and hypoxia, and vascular damage. Somatic effect and genetic damage are two main effects of radiation at the cellular level. A well-illustrated figure on effects and consequences ionizing radiation on deoxyribonucleic acid has been presented in this chapter.