Experimental gerontological research is necessary to obtain optimal information and thus ensure proper drug therapy for the elderly. Most older persons acquire multiple diseases, first of all chronic diseases. They involve complex problems of a physical, social, and psychological nature. The multimorbidity of the elderly raises many questions in drug therapy. By contrast with our extensive knowledge of pharmacokinetics and pharmacodynamics in younger age groups, few facts are available in respect of the elderly. A variety of factors may influence drug therapy. Physiological and pathological age-related changes of molecules, cells, organs, and the total organism may interact to enhance or inhibit drug therapy in higher age groups. It is well known that elderly patients are overmedicated and therefore the incidence of adverse drug reactions increases with age. Elderly patients with multimorbidity often have a diminished body clearance of drugs; age-related changes of the kidney seems to be one of the most important factors in this regard. Far less important than the elimination of drugs through the kidney is their excretion in the bile. So far results have disclosed that oxidative steps in drug clearance are more likely to be disturbed than phase II reactions. Furthermore; changes in distribution volume and age-related alterations in receptor sensitivity also influence the clearance of drugs in the elderly.