While the legal concept of “equity” as a body of rules is routinely employed in a number of fields of international law relevant to the management of water resources and the supply of water services, including international environmental law and international human rights law, it is absolutely central to international water resources law, i.e. the body of rules concerned with the inter-State allocation of rights in the uses and benefits of shared transboundary waters. As suggested by the cardinal principle of international water law, the principle of equitable and reasonable utilisation, the overarching objective of this body of law is to determine how such rights in shared water resources can be allocated equitably, taking account of a range of relevant factors and considerations. Of course, the ultimate aim of the corpus of international rules applying to water resources management is that of ensuring human well-being by achieving the greatest attainable measure of “water security”. However, there exists little consensus about the normative nature or content of the equitable rules or principles to be applied in this context or about their legal implications for the cooperative management of transboundary waters. This chapter attempts to map the use of equitable concepts in cognate areas of international law in order to shed some light on its possible application in the field of international water law.