The Groundwater Component of the Transboundary Waters Assessment Programme (TWAP) deals with Transboundary Aquifers (TBAs) and with Groundwater Systems of Small Island Developing States (SIDS). It was executed by the UNESCO International Hydrological Programme (IHP) and represents the first structured baseline assessment of the state of 199 transboundary aquifers and 42 SIDS groundwater systems as a basis for periodic assessments and to develop scenarios of possible future developments.