With the recent introduction of a number of new immunosuppressive drugs there is an unprecedented interest in therapeutic drug monitoring of immunosuppressive drugs. Most published data on therapeutic drug monitoring in transplantation come from retrospective studies and show a correlation between drug concentrations and toxicity or between concentrations and efficacy. No randomized studies have compared traditional dosing (dose reduction if side effects occur, dose increase if insufficient efficacy is shown) with concentration-controlled dosing. Nevertheless, for a number of immunosuppressive drugs pharmacokinetic monitoring, based on trough levels, is routine practice. Pharmacodynamic monitoring of immunosuppressive drugs has not reached the stage of widespread clinical application. Prospective investigations on the contribution of therapeutic drug monitoring may result in further improvement in the safety and efficacy of our immunosuppressive regimens and more refined methods for therapeutic drug monitoring.