Can calcium and magnesium (“hardness”) in drinking water contribute to preventing disease? This book documents the outputs of an unprecedented group of experts assembled by the World Health Organization to address this question. It includes their comprehensive consensus view on what is known and what is not about the role and possible health benefit of calcium and magnesium in drinking-water. Also included is a series of chapters each authored by internationally renowned experts reviewing the state of the art in different aspects including: global dietary calcium and magnesium intakes; the contribution of drinking water to calcium and magnesium intake; health significance of calcium and magnesium; role of drinking-water in relation to bone metabolism; epidemiological studies and the association of cardiovascular disease risks with water hardness and magnesium in particular; water production; technical issues and economics. In both developed and developing countries, typical diets are often deficient in calcium and magnesium—essential minerals which are necessary for the development of strong bones and teeth, and for cardiovascular function. At the same time, there is evidence that consuming “hard” drinking-water may be associated with reduced risks for some diseases. Climate change and other ongoing changes will increase the use of high tech treatments—for example desalination and reclamation of polluted waters and mean that the issue will be of increasing future importance.