The increase in world population is putting growing pressure on available freshwater stocks, both surface and underground. In particular, the more arid areas of the planet are increasingly dependent on groundwater. The conflict potential of ever scarcer water is all the greater when rivers, lakes or aquifers flow or lie astride the borders separating two or more sovereign riparian or aquifer States. Although aquifers, like rivers and lakes, can stretch beyond the national borders, unlike rivers and lakes they are invisible. Aquifers can be connected to surface water systems, and receive contemporary recharge, or not – in which case the groundwater stored in them is non-renewable, or fossil. In addition, aquifers are particularly vulnerable to pollution and to over-exploitation. Groundwater pollution originates from “point” sources, like the outfalls of industrial and sewerage systems, and from “diffuse” or non-point sources, like the drainage of farmland and inflitration of pesticides, herbicides and fertilizers underground. To make matters worse, pollution of non-recharging aquifers can be irreversible, and damage the aquifer permanently. State practice in the matter of transboundary or shared aquifers is evolving, as attested by the fact that only a handful of them, out of 445 documented transboundary aquifers, are governed by a cooperation agreement among the States concerned. The focus of this paper is on the Nubian Sandstone Aquifer System (NSAS), shared by Egypt, Chad, Sudan and Libya, and on the relevant instruments of cooperation among the four aquifer States. In particular, the attention will be directed at substantive and procedural rules, as well as rules for the resolution of disputes, including their actual functioning, with a view to assessing the intensity and functionality of cooperation among the four concerned States. In the case at hand, cooperation appears to rest mostly on procedural rules. Cooperation among the NSAS countries will be analyzed based on the agreements made by the four countries in a time span of twenty-plus years, from 1992 to-date. Such agreements bear out an evolutionary pattern of cooperation attesting to the will of the four countries to cooperate in the management of the shared aquifer in an incremental fashion, and within the limitations imposed by the embyonic stage of relations among the four countries. Ultimately, this paper aims to assess the effectiveness of cooperation, based on the norms agreed to by the cooperating NSAS States. In the author’s opinion, the NSAS countries should strive to attain a more mature level of cooperation, and in particular an agreement on substantive norms, and on norms regulating the settlement of disputes. The author concludes recommending the principles enshrined in the UN Resolution carrying “The law of transboundary aquifers”, adopted in 2008, as a basis for such agreement.