This report describes the methodology for the assessment of transboundary river basins prepared by the Transboundary River Basins Group (hereafter referred to as the River Basins Group). It is an output of the Medium-Sized Project (MSP) of the Transboundary Waters Assessment Programme (TWAP), with implementation of the assessment to be undertaken in the next phase (Full Size Project – FSP). The MSP is funded by the Global Environment Facility (GEF) and implemented by UNEP.