At high dietary levels, selenium causes adverse effects in animals. Aquatic birds are among the more sensitive wildlife species, because their reproduction can be impaired when dietary selenium levels exceed about 4 µg/g (dry weight basis). Fish can also be adversely affected at similar dietary exposures. Conversely, low dietary levels of selenium (below about 0.1 to 0.5 µg/g) cause nutritional deficiencies in domestic animals, fish, and wildlife. Selenium became recognized as a significant environmental contaminant for wildlife in 1983, with the discovery of developmental abnormalities and excessive embryonic mortality in aquatic birds at Kesterson Reservoir, California. There are a number of environmental settings in which selenium warrants concern and must be considered carefully in relation to potential effects to fish and wildlife, and there may still be surprises in which it unexpectedly becomes a significant ecological issue. However, if selenium is included among the chemicals of potential ecological concern when ecological risk assessments are planned and conducted, there is enough information to thoroughly evaluate its environmental significance in much the same way as other contaminants. The main difference is that it is essential to have a good understanding of selenium's occurrence, complex biogeochemistry, and ecotoxicology to avoid serious errors.