The purpose of this article is to construct a theory of water politics by using the case study of the transnational role and involvement of interest groups in the debate surrounding the proposed Epupa Dam. Interest groups are defined as non-state entities - supported by a specific constituency and converging on an exact issue - which influence government policies and other non-state and inter-governmental institutions, in the national and international political domains. Nowhere is this influence more obvious than in the water politics of water resource management projects (WRMPs), such as hydropower installations. Since the Namibian government announced plans to construct another hydroelectric power plant on the Kunene River in the early 1990s, opposition from interest groups had a significant impact on the political debate regarding the proposed Epupa Dam. To be sure, interest groups did not only focus their lobbying campaign on the national political domain, but they also went transnational in their endeavours to have the project halted. By describing and analysing the role and involvement of interest groups in the water politics of Epupa, a new theory - hydro-normative commensalism - and a definition of water politics are developed. The theory and definition place emphasis on the role of norms in water politics and the way in which norms are used by interest groups and states to advance their arguments in the debate over the proposed Epupa Dam.