Ionizing radiation is ubiquitous in continually changing amounts, as it was from the beginning of life on earth. Those who live in the mountains are immersed in twice as much ionizing radiation as those who live in coastal areas. A most dramatic effect of low doses of ionizing radiation is to make the recipient more resistant to subsequent harmful doses of radiation. Although large doses of ionizing radiation are devastating to dividing cells, as exemplified by harm to fetuses, appropriate irradiation is found to be correlated to increased numbers of young. Animals benefit from low doses of ionizing radiation by exhibiting increased life span, reproductive capacity, disease resistance, resistance to larger doses of radiation, and growth rates. Ionizing radiation is thought to act primarily through a small set of highly reactive chemical radicals. Extensive experiments in plant growth have focused upon practical application of relatively low doses of ionizing radiation for increased production.