Recent investigations of the Physical Agents Effects Branch (PAEB) of NIOSH into the biological hazards of ionizing, optical, and microwave radiofrequency (RF) radiation are reviewed. An estimate of workers potentially exposed to radiation by job and sex is given. Studies of ionizing radiation undertaken by the PAEB are described, which include investigations of radiation doses received by security personnel in airports and by radar unit personnel, radiation transmitted from newspaper office video display terminals, and an alleged increase in leukemia and cancer risk in submarine workers. The results of a PAEB sponsored literature review on ionizing radiation carcinogenesis are discussed. PAEB optical radiation studies, dealing extensively with ultraviolet radiation, are mentioned. Other optical studies concern radiation produced by welding processes, and infrared radiation. A program initiated by NIOSH to investigate the teratogenic effects of RF exposure and efforts to develop a new system for measuring biological doses of RF also are noted.