The North-Western Sahara Basin (NWSAS) comprises two main aquifers: the deep “Continental Intercalaire” (CI), and the “Complexe Terminal” (CT). With a surface area of approximately 1 000 000 km2, the CI extends across three countries, Algeria,Tunisia and Libya and constitutes one of the largest groundwater systems in the world. This resource is generally considered as being “fossil”, i.e. inherited from previous climatic conditions, more humid than at present, with a very limited modern recharge. This basin supplied an estimated volume of 2.2 billion m3 fresh water for domestic water supply, agriculture and other industrial purposes. Groundwater withdrawals from the NWSAS increased from about 14 m3/s in 1950 to 82m3/s in 2000, resulting in decreases in the natural water flows. Over the last two decades, isotopic investigations have been carried out (18O, 14C, 36Cl) and rare gas (He, Ne, Ar, Kr, Xe) to assess the groundwater resource potential in the Sahara of Algeria, Tunisia and Libya. The compilation of isotopic data indicate that waters from CT and CI aquifers are characterised by depleted oxygen-18 and deuterium isotope contents as compared to that of the modern rainfall. This would suggest that the modern rainfall is not recharging these ground waters. Although some sources for active recharge cannot be neglected. Different studies have shown that the NWSAS is recharged by infiltration of surface runoff around the periphery of the domain, particularly around the Saharan Atlas, the Dahar, Tadmait and Tinrhert as well as in the Great Occidental basin during years of exceptional rainfall. The main objective of the present study, is to gather all these data and to examine how they may be interpreted in terms of groundwater residence time, recharge rate, evaporation losses can help the water managers of the three involved countries to develop or refine appropriate models. This should facilitate the implementation of a trans-boundary integrated management of the shared resources.