Abstract: In this paper,the questions,such as what is a GIS (Geographical Information System)?what can a GIS do? and how to select GIS in an application? were discussed.The applications of GIS in hydrology and water resources in last decade were also summarised briefly.Finally,the Decision Support System for Water Management at Catchment scale(DSS-C W M)Based on GIS (the ARC/INFO method),which was developed by the Centre for Water Resources Research,University College Dublin,Ireland,was introduced as a case study.