Selenium (Se) content in soils varies greatly depending upon the parent rock, weathering, pH and texture. In general, total soil Se content of 0.1 to 0.6 mg kg -1 is considered deficient. Selenium deficiency regions in New Zealand, Denmark and the Atlantic Region of Canada contain 0.1 to 0.6 mg Se kg -1 in the soil. Soil acidity is an important factor resulting in decreased Se availability to crops. Selenium concentration in plants can range from 0.005 mg kg -1 in deficient crops to more than 1000 mg kg -1 in Se accumulators on seleniferous soils. Brassicas and legumes, particularly soybeans, contain higher Se than other crops. Selenium fertilization of crops is now permitted in a few countries, such as, New Zealand, Finland and to a limited extent in China, the United States, and Canada. Feed crops containing more than 0.1 mg Se kg -1 should protect livestock from Se deficiency disorders. Selenium is an essential mineral for livestock as well as for humans, but its essentiality for the growth of cultivated crops has so far not been demonstrated. Inadequate Se in animal rations can cause white muscle disease in calves, sheep and goat, exudative diathesis in poultry and mulberry heart disease in pigs. Selenium deficiency can be prevented by Se injections to females at late gestation and/or to the young stock shortly after birth. In humans, lack of Se has been linked to several kinds of cancer, heart disease and other chronic and life threatening conditions. Adequate Se nutrition supports efficient thyroid hormone synthesis and metabolism and protects the thyroid gland from damage by excessive iodide exposure. Inadequate plasma Se can adversely affect the maintenance of optimal health. Selenium appears to play a key role in health maintenance of aging individuals. Oral Se therapy has been reported to produce significant decreases in lung, prostate and colorectal cancer. Selenium has been shown to help prevent cardiomyopathy in young children in China. Pills containing Se alone or in combination with vitamins and or minerals are available in several countries as human supplements. Eating Se enriched foods and animal products and/or Se pills may protect humans from pathology associated with Se deficiency.