Immunosuppressive drugs make possible the acceptance of organ allografts among individuals with differences in Major Histocompatibility Antigens (HLA). Transplantation of vital organs prolongs the survival of patients with terminal diseases, and this procedure has become a routine practice in the clinic, mainly because of advances in immunosuppressive therapy. Some immunosuppressive drugs, such as glucocorticosteroids and azathioprine, have been used for the past 30 years. More recently, newly discovered agents with a better ratio of efficacy to toxicity have been added to the armamentarium of anti-rejection therapies. Progress in understanding T cell activation in response to alloantigens has contributed to the development of new and more selective strategies to control the immune response and prevent acute rejection. The use of drugs in combination, with or without monoclonal antibodies, has also improved the efficacy and reduced the toxicity of immunosuppressive therapies. The new agents include drugs that interfere with calcineurin, inhibitors of de novo purine biosynthesis, kinase inhibitors, as well as monoclonal antibodies that block activation signals on the surface of T cells or co-stimulatory signals between T cells and antigen-presenting cells. In this review the modes of action of commonly used immunosuppressive drugs are described. Successful new strategies are also being developed to establish tolerance to allografts in rodents and non-human primates. The progress in these approaches, although still in the experimental stages, offers promising alternatives for these patients in the future. Treatment protocols using combinations of drugs with antibodies that might produce tolerance in humans are also discussed.