This book focuses on water as a determinant of human livelihood and for safeguarding environmental security. We have analysed water availability over the world’s continents, in absolute terms and in relative terms in a per capita dimension. We also distinguished between the blue water flows available for direct withdrawal and use, and the green water flows linked to terrestrial ecosystems, including crop production and forestry. World maps highlighted fundamental differences in terms of per capita blue water flow, and clarified that certain regions are much more vulnerable than others.