This Analytical Brief serves as a starting point for dialogue on water security in the United Nations system. This Brief offers a working definition of water security developed from contributions made by the broad range of organizations, agencies, programmes and institutions that form UN-Water. Through this Brief, UN-Water aims to capture the constantly evolving dimensions of water-related issues, offering a holistic outlook on challenges under the umbrella of water security. It highlights the main challenges to be addressed, the role water security plays in policy agendas, and possible options for addressing water security challenges. A working definition of water security provides a common framework for collaboration across the UN system. Water security is defined as the capacity of a population to safeguard sustainable access to adequate quantities of acceptable quality water for sustaining livelihoods, human well-being, and socio-economic development, for ensuring protection against water-borne pollution and water-related disasters, and for preserving ecosystems in a climate of peace and political stability. This shared definition will facilitate the work of UN-Water and its Members and Partners.