In the past, postmarketing research, postmarketing surveillance and pharmacovigilance were synonymous with phase IV studies because the main activities of the regulatory agency (e.g. FDA) were focused on the monitoring of adverse drug events and inspections of drug manufacturing facilities and products. (1) However, the fact that not all FDA mandated (classical phase IV trials) research consists of randomized controlled trials (RCTs), and not all postmarketing activities are limited to safety issues (pharmacovigilance), these terms require clarification. This chapter attempts to clarify the confusing terminology; and, to discuss many of the postmarketing research designs-both their place in clinical research as well as their limitations. © Springer Science + Business Media B.V. 2008.