Between August 2014 and January 2018, the joint Namibian-German research project OPTIMASS explored the different links between geo- and biosphere along a rainfall gradient across Namibia from north to south. Funded by the German Federal Ministry of Education and Research (BMBF), OPTIMASS aimed at providing science-based information that will assist land users and decision makers towards a sustainable management of these important and beautiful ecosystems. The project was jointly coordinated by the University of Potsdam, Germany and the University of Namibia, with further partners from the Namibia University of Science and Technology, Freie Universität Berlin, Universities of Tübingen and Hohernheim, and the Institute for Social-Ecological Research in Frankfurt/Main. Focusing on the interplay between the abiotic and the biotic world, OPTIMASS combined farmer interviews, field research, experiments and the development of novel computer simulation models. OPTIMASS scientists recorded current land use practices and analysed feedbacks between soil, water, vegetation and biodiversity explicitly con- sidering the role of plant physiology and soil digging animals. This knowledge can be used to inform policy makers and to develop guidelines for sustainable management of water and rangeland under predicted changes in climate. The booklet at hand summarizes the key findings of OPTIMASS and presents exemplary studies in more detail. The project came along with a strong educational component and enabled joint PhD-, Master-, Honours- and Bachelor-projects of Namibian and German students. In annual summer schools alternately held in Namibia and Germany, researchers of all involved institutions shared their knowledge about state-of-the art methodology, savanna rangeland ecology and hydrology with many students.