The Okavango River Basin Commission, OKACOM, initiated a project titled the Environmental Protection and Sustainable Management of the Okavango River Basin (EPSMO). This was approved by the United Nations Development Program (UNDP), to be executed by the United Nations Food and Agriculture Organization (FAO). The standard UNDP process is a Transboundary Diagnostic Analysis followed by a Strategic Action Programme of joint management to address threats to the basin's linked land and water systems. Because of the pristine nature of the Okavango River, this approach was modified to include an Environmental Flow Assessment (EFA). To complete the EFA, EPSMO collaborated with the BIOKAVANGO Project at the Harry Oppenheimer Okavango Research Centre of the University of Botswana, in 2008 to conduct a basin-wide EFA for the Okavango River system. This is report number 4 in the report series for the EFA. It outlines the location and main characteristics of the Okavango River Basin, the river and delta zones recognised by hydrologists, geomorphologists, aquatic chemists, biologists, sociologists and resource economists, and how these were harmonised to produce Integrated Units of Analysis (IUAs). The process of selection of IUAs and sites to represent the whole basin during the EFlows work is then described.