Geographical Information System (GIS) is used to capture, visualise, process, and evaluate spatio-temporal data. GIS is an extremely versatile instrument that can be applied to support various decisions related to water resources management – mapping land use, measuring catchment areas, real time flows during floods, measuring access of potable water, etc. This Tool describes in detail the two key data models in GIS and discusses the potential of GIS towards supporting IWRM.