Namibia has high levels of diversity and endemism, and is recognised as a priority region for conservation . Ants are important components of arid ecosystems, they are ecologically dominant and drive vital processes, but there are few studies of them in Namibia. Ant-plant interactions form a significant area of ecological research; a classic example being the swollen-thorn acacias of Africa. This thesis describes Namibian ground and arboreal ant diversity and dominance, and characterises the ecology of a novel ant-acacia mutualism. I demonstrate that ant species richness and composition is highly variable across three arid Namibian habitats; saltpan, savannah and desert. Arboreal ants are shown to be a unique component of diversity, despite lower species richness than ground ants. The abundance of dominant ants influences species richness on the ground, but not on vegetation. In contrast to most ant-plant systems, individual camelthorn acacia trees, Vachellia erioloba, are stably and simultaneously inhabited by multiple ant species that nest within swollen-thorn domatia. The ant community on V. erioloba provides defence against herbivorous insects, although their effectiveness varies with herbivore identity. Nest site selection by the four taxonomically diverse ant species on V. erioloba is based on domatia morphology. Colony size increases with domatia size, but each species responds differently to availability of nesting space through varying levels of investment in brood production. Ants inhabiting myrmecophytes are thought to be nest-site limited and under intense competition. Unusually, on V. erioloba this is not the case, which may facilitate the rare coexistence of multiple ant species on individual host plants in this system. My research highlights the need for further work on ant diversity and ant-plant mutualisms in Namibia. The implications of this work are discussed and directions for future research suggested.