This guide is a short introduction for decision-makers and researchers or anyone else considering whether a systematic review may be appropriate to fill a gap in knowledge or to use as a resource. It will help anybody planning on commissioning a review of what research is already out there. You may be an analyst, evaluator, policymaker or commissioner. The guide is aimed at anybody from central government, local authorities, public service providers, regulatory and advisory bodies, charities or the consultancy sector. We also hope it will inform staff engaged in developing a ‘What Works’ centre; synthesizing research is a core principle of the new independent ‘What Works’ institutions.