The electromagnetic spectrum encompasses frequencies that range from above approximately 10²⁰ hertz (Hz) for ionizing radiation at the high end of the spectrum to static fields and power frequencies of 50–60 Hz at the low end. In order of decreasing frequency, the spectrum comprises gamma-rays, X-rays, ultraviolet radiation, visible light, infrared radiation, microwaves, radio waves and extremely low frequency electric and magnetic fields (ELF EMF). This chapter reviews sources and health effects of human exposure to radiation within the electromagnetic spectrum, and for ionizing radiation exposure to subatomic particles, as well as reviewing policies for limiting human exposure where appropriate.