The water systems of the world - aquifers, lakes, rivers, large marine ecosystems, and open ocean - support the socioeconomic development and wellbeing of the world's population. Many of these systems are shared by two or more nations and these transboundary resources are interlinked by a complex web of environmental, political, economic and security interdependencies. In order to address this challenge UNEP, under the auspices of the GEF, coordinated over a 2 years period from 2009 to 2011 the implementation of the Medium Size Project (MSP) entitled \"Development of the Methodology and Arrangements for the GEF Transboundary Waters Assessment Programme (TWAP)\". This document – Volume 1 – is a summary of the detailed methodologies described in volumes 2 - 6.