This chapter describes the deleterious effects of radiation in humans causing cancer and genetic defects that result from DNA and chromosome damage by radiation. A brief description of cell structure and cell growth is given. Sources of radiation exposure to the US population are given. Acute short-term effects from whole body radiation are presented. Long-term effects including carcinogenesis and genetic effects are also provided. The LNT theory for risk estimate from radiation exposure is discussed as opposed to the theory of a threshold for risk. A discussion is in order on the effects of radiation on the embryo and fetus. A pertinent discussion is presented on the risk versus benefit of nuclear and radiological procedures.