This report provides a conceptual framework to address food security under conditions of water scarcity in agriculture. It has been prepared by a team of FAO staff and consultants in the framework of the project "Coping with water scarcity: the role of agriculture?", and has been discussed at an Expert Consultation meeting organized in FAO, Rome in December 2009 on the same subject. It was subsequently edited and revised, taking account of discussions in the Expert Consultation and materials presented to the meeting. The document offers views on the conceptual framework on which FAO's water scarcity program should be based, proposes a set of definitions associated with the concept of water scarcity, and indicates the main principles on which FAO should base its action in support to its member countries.