Household water insecurity is a pressing problem in developing countries. Unsustainable water withdrawal is increasing due to population growth, industrialization, urbanization, and increasing agricultural production which leads to various problems. The number of countries facing problems of water scarcity and insufficient water supply is rising. Already there are 1.2 billion people without access to clean water, many of whom live in 20 developing countries classified as ‘water scarce’. Typically it is found in these countries, that the poor pay particularly high prices for water and are most water insecure. Progress towards water security can be made only if there is a more comprehensive understanding of the interactions among waters’ various characteristics and functions. Water is not only a natural resource, but also an economic commodity, and a human consumption good or entitlement. The problems of water insecurity can be grouped under three main headings: availability, access and usage. In the framework of a multidisciplinary approach to the analysis of water problems, the paper elaborates on these three elements, defining sectoral and cross-sectoral knowledge gaps. The paper concludes with a research agenda in support of improved policy design and action.