To appreciate the water issues that will bother greater and greater fractions of the world's population as we move into the 21st century, it is important to develop some feeling for two basic, and closely related, elements of the situation, demand and supply.

To meet the 'Demand' for water, however this term is understood, planners have to seek alternative sources and 'develop' new supplies. With all close and convenient sources now near full exploitation, it is the costs of water development, transportation and distribution, rather than any real scarcity, that will that increasingly constrain water supply initiatives in the years ahead.

To understand the nature of the world's water 'problem' requires a perspective on the balance between resources and current water use. As the level of use approaches the available resource, water stress and scarcity can be anticipated. Evaluating the effect of this sort of stress requires appreciation of the fact that 'water' is not all and everywhere the same. From a human use perspective there are in fact - several different kinds of water, and this is particularly important in relation to the emergence of water markets.

It is only after developing some understanding of these several aspects of demand and supply that one can begin to gain some perspective on the question of 'water balance' around the world.