This thesis deals with the enzyme Indoleamine 2,3-Dioxygenase (IDO) which was originally described as an inhibitory mechanism of facultatively intracellular pathogens. By now, it is known that IDO also has a lot of other functions. Not all of them are clearly established at the moment. IDO inhibits the proliferation of T lymphocytes through the degradation of the essential amino acid tryptophan. Furthermore, IDO is associated with a series of diseases. Thus it is known, that some tumors express IDO as a mechanism to overcome the immune defense of the host. The enzyme also seems to be involved in the pathogenesis of Human Immundeficiency Virus (HIV). For example, an in vitro infection of human macrophages with HIV leads to a high expression of IDO. Because of its potential for immune-tolerance induction, IDO is an interesting target for future therapies. In this thesis I am giving an overview of the current state of research about regulation, function and effects on T lymphocytes of IDO.