This paper reviews the growing literature dealing with the antagonistic effect of selenium (Se) compounds on the toxicity of mercury (Hg) compounds in higher animals and organisms present in the aquatic environment. It covers both laboratory and field studies and summarizes the possible mechanisms that explain the protective action of Se compounds on mercuric mercury (Hg2+) and methylmercury (CH3Hg+) toxicity. The review is subdivided according to the molecular form of Hg and the organisms in which the antagonism has been studied. Many authors suggest that the protective effect of selenite on the toxicity of Hg2+in mammals is due mainly to the in vivo formation of mercuric selenide (HgSe), a stable and biologically inert complex. The detection of HgSe has been confirmed in several studies in support of this mechanism. Possible mechanisms that may be involved in the antagonism between Se compounds and CH3Hg+in mammals and aquatic organisms are also presented. The possibility of adding Se compounds to contaminated lakes and reservoirs as a remediation technique to limit the bioaccumulation of Hg2+and CH3Hg+is critically discussed.