The principle of equitable and reasonable utilization is widely accepted as one of the fundamental principles of international water law and is additionally regarded as a guiding principle in utilization of shared natural resources in general. Underground geological formations, known as (transboundary) aquifers contain groundwaters that make up approximately 97% of all the non-frozen fresh water resources available on the planet. In result, groundwater is the single most extracted natural resource in the world. Despite mankind’s immense reliance on this resource, States only occasionally enter into international agreements regulating the utilization of transboundary aquifers as they rarely possess adequate scientific and technical information to do so. Furthermore, States often regard aquifers and groundwaters contained therein as subject to their exclusive territorial sovereignty. This raises questions if and to what extent is the principle of equitable and reasonable utilization applicable to transboundary aquifers. In providing answers, the Thesis does not merely define the principle of equitable and reasonable utilization of transboundary aquifers, but takes a multidimensional approach. To this end, it places the principle into a wider legal framework by overviewing relevant international agreements and by discussing the possible limitations to State sovereignty over shared water resources. Simultaneously, the Thesis suggests that the peculiarities of groundwaters require a sui generis legal regime, different from the one applicable to surface water resources. Where relevant, special consideration is also given to the jurisprudence of (international) courts and tribunals, with special emphasis on the case-law of the International Court of Justice.