Water is the most abundant environmental resource, covering more than 70 per cent of the Earth’s surface. Most of this (more than 97 per cent) is salt water and this chapter is concerned only with the residual balance of fresh water (see Chapter 10 for a discussion of marine resources). In fact, most of this residual is not available for human use and estimates of the global water balance suggest that more than 75 per cent of it is ‘locked up’ in the polar ice caps. (McDonald and Kay, 1988). Overall, the rivers and freshwater lakes of the world account for an estimated 0.02 per cent of the water on the planet with groundwater and soil moisture contributing a further 0.58 per cent and the atmosphere an infinitesimal 0.001 per cent. Expressed in these terms the apparent abundance of fresh water begins to appear less impressive and only 0.6 per cent of the total volume of water on the planet is available in this form. This resource is uniquely versatile and is used for drinking, to sustain agriculture and industry, to serve as a means of transportation and waste disposal and as a focus for a wide range of recreational activities. This list is by no means exhaustive and the multiple uses of water not only place pressures upon available supplies, but also create serious conflicts of interest between, for example, the requirements of industry and those of amenity groups. Such conflicts often revolve around the inter-relationships between the two key attributes of water quantity and water quality. These attributes are considered separately in this chapter before turning to some of the management issues involved in coordinating the multiple uses of water.