Wildlife may be exposed to mercury (Hg) and methylmercury (MeHg) from a variety of environmental sources, including mine tailings, industrial effluent, agricultural drainwater, impoundments, and atmospheric deposition from electric power generation. Terrestrial and aquatic wildlife may be at risk from exposure to waterborne Hg and MeHg. The transformation of inorganic Hg by anaerobic sediment microorganisms in the water column produces MeHg, which bioaccumulates at successive trophic levels in the food chain. If high trophic level feeders, such as piscivorous birds and mammals, ingest sufficient MeHg in prey and drinking water, Hg toxicoses, including damage to nervous, excretory and reproductive systems, result. Currently accepted no observed adverse effect levels (NOAELs) for waterborne Hg in wildlife have been developed from the piscivorous model in which most dietary Hg is in the methyl form. Such model are not applicable to omnivores, insectivores, and other potentially affected groups, and have not incorpotated data from other important matrices, such as eggs and muscle. The purpose of this paper is to present a comprehensive review of the Hg literature as it relates to effects on wildlife, including previously understudied groups. We present a critique of the current state of knowledge about effects of Hg on wildlife as an aid to identifying missing information and to planning research needed for conducting a complete assessment of Hg risks to wildlife. This review summarizes the toxicity of Hg to birds and mammals, the mechanisms of Hg toxicity, the measurement of Hg in biota, and interpretation of residue data.