Evidence Gap Maps (EGMs) provide an important tool for evidence-informed policymaking and strategic research prioritisation. 3ie EGMs are collections of evidence on the effects of development policies and programmes in a particular sector, sub-sector or thematic area, structured around a framework of interventions and outcomes. They provide a graphical display of existing and ongoing systematic reviews and impact evaluations. As EGMs gain in popularity as a decision-making tool, it is important to know how they are being produced. As yet, there is no internationally accepted definition or set of methods for doing them. This paper provides a conceptual introduction and demonstration of 3ie EGMS to help clarify how and why we produce them the way we do. Later in 2017, 3ie will be publishing a manual and checklist for using the 3ie approach to produce an EGM.