Immunosuppressant drugs are crucial in order to protect transplanted kidney, heart or liver against the body's natural defence mechanisms. These drugs are used to prevent both acute rejection of the organ as well as chronic deterioration of the graft over longer periods of time. Currently used immunosuppressant drugs are calcineurin inhibitors (cyclosporin and tacrolimus), steroids (prednisolone and methylprednisolone), antimetabolites (azathioprine and mycophenolate mofetil), antiproliferatives (sirolimus) and monoclonal antibodies that are used as an induction therapy (basilximab, daclizumab and muromonab). Over time, we have learnt more about existing immunosuppressive choices and the ways to monitor these drugs, and the current trend in immunosuppression therapy is towards tailoring the therapy according to an individual patient. The major issue that is now emerging is not how to prevent acute rejection, as current drugs are all efficacious, but rather how to avoid the long-term side effects that can harm both the graft and host, and negatively influence compliance. The market in transplantation is considerable and still growing; the whole market including the immuno-suppressants used in autoimmune diseases had sales of approximately pound 1.5 billion in 2001. There are currently 23 new drugs in advanced clinical development intended to be used in either organ transplantation or as treatment of autoimmune diseases. The new drugs that are intended to be used in transplantation are mostly analogues of currently used drugs with improved safety and pharmaceutical profiles.