Groundwater is a classic illustration of Eric Zimmerman’s assertion during the 1930s that ‘A resource is not, it becomes’ (Zimmerman, 1933). Something becomes a resource when it is obtainable and has an economic demand that can be profitably met. Groundwater, all of 23 million BCM the earth has,1 has been with us for millennia. Yet, just a tiny fraction has been used for domestic needs and garden agriculture through shallow open wells as well as thousands of miles of ‘water galleries’ (qanats2) from Kerala in India to Iran to Spain and Tunisia. True, not all groundwater is easily usable and thus a ‘resource’. Hence, globally, we used hardly any of it until a century ago when tube wells, motor pumps and fossil energy made it techno-economically viable to access it in large volumes. Even then, demand for groundwater in agriculture began soaring only when irrigation boosted farm productivity under the Green Revolution technology. During recent decades, groundwater has emerged as the mainstay of drinking water supply in countless cities and settlements throughout the world. Yet, agricultural groundwater use has accounted for over four-fifths of it, and this has experienced vigorous growth, especially in populous agrarian economies like India, China, Pakistan and Bangladesh (see Fig. 1). Over the past 50 years, groundwater development has added some 120 million hectares of irrigated areas, mostly in arid and semi-arid regions of the world.