Evidence-gap maps present a new addition to the tools available to support evidence-informed policy making. Evidence-gap maps are thematic evidence collections covering a range of issues such as maternal health, HIV/AIDS, and agriculture. They present a visual overview of existing systematic reviews or impact evaluations in a sector or subsector, schematically representing the types of interventions evaluated and outcomes reported. Gap maps enable policy makers and practitioners to explore the findings and quality of the existing evidence and facilitate informed judgment and evidence-based decision making in international development policy and practice. The gap map also identifies key "gaps" where little or no evidence from impact evaluations and systematic reviews is available and where future research should be focused. Thus, gap maps can be a useful tool for developing a strategic approach to building the evidence base in a particular sector. This paper provides an introduction to evidence-gap maps, outlines the gap-map methodology, and presents some examples.