This study focuses on the transnational role and involvement of interest groups in the water politics of two Southern African international river basins the Kunene and Orange Rivers. The thesis is in part based on the theory of social constructivism, with the purpose of investigating the extent to which the collective transnational activities of interest groups, regarding the implementation of WRMPs in selected Southern African international rivers, lead to the undermining of the acceptance of the actions and policies that are authorised at the state level of world politics. Two case studies were analysed namely the proposed Epupa Dam and the Lesotho Highlands Water Project (LHWP). Regarding the problem statements and the chosen theoretical approach, a number of findings were made. Firstly, interest groups posed a substantial challenge to the national and international agential power of the states., although the erosion of the policies initiated at the state level differed in respect of the two cases. In the Kunene River basin, the interest groups have moderate international agential power and moderate to high domestic agential power . Namibia's international and domestic agential power is substantially lower than that of the interest groups, mainly on account Angola opposing plans for a dam at Epupa. In the Orange River basin, the interest groups have high domestic agential power, especially the Lesotho interest groups, and moderate international agential power . The reason for the high domestic agential power of the Lesotho-based interest groups is Lesotho's reflexive agential power . This reflexivity is a direct consequence of Lesotho's changing identity from a politically unstable to stable state. South Africa's international and domestic agential power is higher than that of the interest groups, mainly because the South African government went ahead with plans to construct the LHWP in an era where there was little opposition to the project. Since there was no interest group involvement during the planning phase of the LHWP from 1956 to 1986, there was no interest group challenge of the LHWP. Also, the interest groups could not effectively challenge the economic power of South Africa. Furthermore, the ANC changed its stance from being anti-LHWP in the 1980s to pro-LHWP when it became the ruling party. This meant that the Project was backed by the ruling party's ideological power. Thus, it was concluded that interest groups had a significant role and influence on the water politics regarding the WRMPs in the international river basins of Southern Africa. The thesis contributes to the body of research on water politics in a number of ways. First, the study contributes to an understanding of the reasons why interest groups are transnationally involved in water politics and of the roles they play in this process. Second, a theory of water politics (hydro-normative commensalism) was developed, that primarily focuses on the role of norms in water politics. Third, a new definition of water politics was developed, stating that water (hydro- ) politics is the transnational interaction, through norm creation and utilisation, between a plethora of non-state and state actors, varying from individuals to collectivities, regarding the allocation and use of, and perception towards domestic and international water resources. The relationship between the state and interest groups is increasingly transnational because of a diminishing capacity of the state to insulate itself from the influences of non-state actors regarding the implementation of policies.