Tryptophan has been acknowledged as an essential amino acid for many years, but only in recent years unique biological and metabolic properties have been attributed to this amino acid. Indeed, tryptophan is now known to be involved in regulatory control mechanisms of the central nervous system and of the liver. Thus, in addition to being an essential building block in proteins, tryptophan has other important effects that influence and regulate vital biological mechanisms. Indepth understanding of how one essential nutrient, such as tryptophan, acts in normal as well as in disease states is of importance. Such information should offer an opportunity whereby specific nutritional manipulation of a dietary component may be used in a rational way in the possible prevention and/or treatment of certain disease states.