Leucaena leucocephala is one of the fastest-growing leguminous trees. Its foliage is used as animal feed, and its leaves and seeds are used as human food in Central America, Indonesia, and Thailand. Mimosine, the toxic, non-protein amino acid in Leucaena, causes alopecia, growth retardation, cataract, goitre, decreased fertility, and mortality in non-ruminants. The mechanism of this toxicity is complicated. Mimosine probably exerts its toxic action by blocking the metabolic pathways of aromatic amino acids and tryptophan; by chelating metals; by antagonizing the action of vitamin B6; by inhibiting DNA, RNA, and protein synthesis; by exerting adverse effects on collagen biosynthesis; and by interfering in the metabolism of some amino acids, primarily glycine. Besides mimosine, other anti-nutritional and toxic factors in L. leucocephala add to its toxicity. Heat, moisture, chemical treatments, ensiling, rotation feeding, cutting management of the plant, new hybrids, introducing micro-organisms into the rumen of ruminants that are unable to detoxify mimosine, and preparing protein isolate from Leucaena seeds have all been used to overcome mimosine toxicity.