Databases are electronic filing systems that have been set up to capture patient data in a variety of clinical and administrative settings. While randomised controlled trials are the gold standard for the evaluation of healthcare interventions, electronic databases are valuable research options for studies of aetiology and prognosis, or where trials are too expensive/not logistically feasible. However, databases exist in many different settings and formats (often developed for administrative or financial reimbursement purposes rather than clinical research), and researchers need to put careful thought into identifying and acquiring relevant data sets. Accuracy of records and validation of diagnoses are key issues when planning a database study. High-quality databases can readily capture outcome data (as part of routine clinical care) without the costs and burden of additional trial-related follow-up, and there are promising hybrid models which combine the benefits of randomisation with the efficiency of outcome ascertainment using existing databases.