Selenium (Se) is an essential element for human health in trace amounts but is harmful in excess. Most plants contain a fairly low Se and crop Se supplements ensure adequate levels for human nutritional needs. Food is the primary source of Se for humans and due to differences in eating habits, its intake varies considerably. In the human body, selenium deficiency can lead to diseases of the endocrine, musculoskeletal, cardiovascular, reproductive, nervous and immune systems. Selenium is an important ingredient in glutathione peroxidase, the main cellular antioxidant enzyme, which can convert free radicals into peroxides, while vitamin E removes free radicals and neutralizes their potentially harmful effects. Excessive amounts of selenium in the human diet are considered toxic, causing liver and kidney damage, blood clotting, heart and liver necrosis, skin lesions, nausea, vomiting, and loss of hair and nails. Semiconducting properties of Se make it of special value for industry. Selenium is a rare element on the planet, and is a non-renewable resource due to its non-efficient and difficult recycling. Except of coal, which is commonly enriched in Se, there are no ores which could be mined for it. Herewith, the world's scarce Se resources need a careful management, monitoring, recycling, and stockpiling for future generations. The first part of this review outlines selenium concentrations in soil, water, and plants in terms of essential and toxicological effects on animals and humans, while the second part briefly overviews novel biotechnological methods of bioremediation of environmental selenium.