This chapter shows how to conduct a comprehensive, objective and reproducible search for studies. It can be the most time-consuming and challenging task in preparing a clinical question for a project or a systematic review. Yet it is also one of the most important. Identifying all relevant studies, and documenting the search for studies with sufficient detail so that it can be reproduced, is after all, largely what distinguishes a systematic review from a traditional narrative review in evidence-based medicine. This chapter explains how, and where, the reviewers should look for studies that may be eligible for inclusion in The Cochrane Library, MEDLINE, EMBASE and other relevant databases that identify appropriate MeSH terms (Medical Subjects Headings). Although currently it is necessary to search multiple sources to identify relevant published studies, it is envisioned that the Cochrane Central Register of Controlled Trials (CENTRAL) in The Cochrane Library will become a comprehensive source for published studies, thus reducing the searching burden for authors. Identifying ongoing studies, however, will continue to remain a challenge until a comprehensive, searchable, ongoing trial register is produced to track, organise, and disseminate reports for ongoing studies, as CENTRAL in The Cochrane Library does for reports of studies that have been published.