The difficulty of accurately identifying and classifying inpatient adverse drug events (ADEs) was first recognized nearly a half century ago. This systematic review sought to evaluate the accuracy of trigger tools, an increasingly common technique used to screen electronic databases for evidence of ADEs. Triggers have been used in this fashion to identify ADEs from inpatient laboratory systems and outpatient electronic health records. This review found that the overall performance of electronic ADE detection systems was poor, and the quality of the studies was limited by variations in ADE definitions and failure to use gold standard methods for validating ADEs. Although they are a promising method for identifying ADEs promptly, the review concludes that electronic triggers still have serious limitations.