With 22 genera and approximately 200 species the Cyperaceae form the third largest flowering plant family of Rwanda and Burundi. These species can be found in almost all habitats, from Lake Tanganyika to the tops of the Virunga Mountains, often with a strong ecological importance. In Rwanda and Burundi two endemic species occur in the Albertine Rift region2. For Rwanda a Cyperaceae key was already available1 but needed to be updated. There is no flora for Burundi yet. Because Rwanda and Burundi are biogeographically very similar the species of the two countries are combined in one key. Differences between the two countries can be found in the Virunga region (Afro-alpine region in the north of Rwanda) and the Mosso (Zambesic region in the south of Burundi). This study is based on 2.456 specimens from the herbaria of Ghent University (GENT) and the National Botanical Garden of Belgium (BR). The keys were tested by botany students of Ghent University. The key represented here forms an important working tool for floristic, fytosociological and ecological research, especially of the wetlands in these countries. In the future an interactive version of the key will be available on the internet.