Organ transplantation has become an important additional option for patients with organ failure. Immunosuppressive drugs showing a very narrow therapeutic window have to be administered. Different transporters and metabolic pathways are responsible for absorption and metabolism of these drugs; for instance, the P-glycoprotein (P-gp) pump regulates the absorption of a drug, and its metabolism is catalyzed by cytochrome P450s (CYPs). As the phenotypes of P-gp or the CYPs are predetermined by their genotypes, genetic testing prior to drug therapy may help to predict the drug doses required. This review describes polymorphisms of the genes coding for P-gp and CYPs, and focuses on the compounds cyclosporin and tacrolimus. It is hoped that this information might help to judge the value of pharmacogenetic testing prior to immunosuppressive therapy in solid organ transplantation.