This article addresses the different approaches to access to water and sanitation by two separate bodies of international law, i.e. water law and human rights law. It shows the slow evolution of the former from a purely inter-State economic dimension, dealing with competing sovereign claims over transboundary watercourses, to one concerned also the needs of peoples and individuals. In light of these developments at the customary level, it illustrates the significant progress in the field at the conventional and Pan-European level represented by the 1999 UN/ECE London Protocol on Water and Health. An analysis of the latter is carried out against the background of the authoritative interpretation of Articles 11 and 12 of the 1966 UN Covenant on Economic, Social and Cultural Rights contained in the 2002 General Comment 15 by the Committee of the Covenant. This comparative analysis shows the full complementarity and, in many respects, even the coincidence between those instruments. From a normative structural point of view, the common denominator between them appears to lie in the due diligence nature of their main obligations enhancing the state legal accountability for water services.