The limitations of pre-marketing clinical trials are analyzed and the need for ongoing drug research after approval is emphasized. Rare adverse drug reactions constitute the most important but not the only question. In addition to so-called post-marketing Drug Utilization Observation studies and spontaneous event reporting systems, other pharmacoepidemiologic study approaches like comparative cohort studies and case control studies, and also randomized clinical trials, are necessary to meet the multitude of scientific objectives of post-approval drug research.