At the Millennium Summit in September 2000, the largest-ever gathering of world leaders adopted the UN Millennium Declaration, setting out the Millennium Development Goals, including "MDG 7: Ensure Environmental Sustainability" which included the target of reducing by half the proportion of people without sustainable access to safe drinking water. This monograph looks at access in terms of five key components and integrates them into the Index of Drinking Water Adequacy - IDWA for short. The substantive papers in the monograph comprise international and intra-national explorations based on IDWA estimates. Globally IDWA estimates for 144 economies have strong correlations with Human Development and Human Poverty Indexes and thus affirm the importance of investment in safe drinking water.