Along with Volume II (Annexes on each of the four sub-basins of the ENB), this Report presents the first trial version of the One System Inventory (OSI) information on the Eastern Nile. Meant to serve as a regional knowledge base across Egypt, Ethiopia and Sudan, to support its multipurpose development objectives, this compiled and synthesized baseline information on water resources, environment and socio-economic status, on each of the four main trans-boundary sub-basins of the Eastern Nile: Abbay-Blue Nile, Baro-Akobo-Sobat-White Nile, Tekeze-Setit-Atbara, and Main Nile. The sub-basin level information was to help decision-makers and senior program and project coordinators for proper planning of natural resources and for further investigative research under the EN Joint Multi-Purpose Program. This is an updated and revised Report, building on previously compiled national and thematic OSI reports on environmental, socio-economic and water resources and synthesis reports of 2007, and benefiting from comments and suggestions received from regional workshops and meetings.