Pharmacoepidemiology is the application of epidemiological principles and methods to the study of drug effects in human populations. The goal of this discipline is to characterize, control and predict the effects and uses of pharmacological treatment modalities. Pharmacoepidemiology is also concerned with the economic impact and health benefits of unintended drug effects. The increasing importance of pharmacoepidemiology has been created by the need to develop a more accurate portrait of how drugs are used in the general population. Sophisticated and potent drug therapies require surveillance beyond the scope of the carefully controlled clinical trials of Phases I, II and III. Case-control and cohort studies, which allow scientists to evaluate the effects of patient variables on clinical outcomes, provide a wealth of information regarding the study of unexpected drug effects, drug utilization, treatment costs and the individualization of therapy.