Radiation includes a great variety of rays and particles that have tremendously different energies. Within this spectrum is a group of rays and particles with enough energy to fragment atoms and molecules into ions and free radicles; this is ionizing radiation. Mixed radiation sources such as cosmic rays include secondary radiation generated when galactic rays hit the earth’s atmosphere. The law of hormesis states that subharmful quantities of any potentially harmful agent may stimulate an organism. The most remarkable aspect of hormesis is that low doses of harmful agents can produce effects that are diametrically opposite to the effects found with high doses of the same agent. Radiation hormesis provides the basis for appropriate utilization of ionizing radiation as a useful tool in our technologic society. It can provide more efficient use of resources, maximum production of grains, vegetables, and meat, and increased health and longevity.