In this chapter, an attempt has been made to discuss the major immunosuppressant agents in use today as well as some of those whose future is as yet undecided. Obviously, not every immunosuppressive modality could be included in this limited space. However, we have endeavored to include agents of practical and theoretical importance to the practicing physician. The therapies discussed here are all hampered by significant degrees of toxicity, which compromises their usefulness and limits their potential. The decision to begin therapy with these agents must be made carefully, and it is incumbent upon the physician to include the patient and patient's family in this process. In many cases, these choices can be based on well-controlled trials in which efficacy has been established. Unfortunately, for many immunologically mediated processes no such studies are available, or those that have been reported are flawed. In such cases, decisions become increasingly difficult. One must then weigh the potential adverse effects of the agent against the possible long-term consequences of witholding such treatment. We can only hope that the continued study of these diseases and the immunosuppressant agents in concert with the development of new, less toxic therapies will make these choices less difficult in the future.