Significant number of accidental water pollution incidents occurred during the past decades in the Danube River Basin, often having transboundary impacts. Growing needs arose at the important water users (first of all at drinking water intakes) along the rivers to get early warning about such incidents. The Danube Accident Emergency Warning System (DAEWS) was put into operation in April 1997, as a result of the short term priority action of the Environmental Programme for the Danube River Basin. The Danube Accident Emergency Warning System provides immediate information on sudden changes in water characteristics, like accidental river pollution incidents having transboundary effects, to assist the responsible authorities and water users in the downstream countries to make preventive measures in time. The countries where the system is now operational are (in the order of their location in the river basin): Germany, Austria, Czech Republic, Slovakia, Hungary, Slovenia, Croatia, Romania, and Bulgaria. The National Centres of the system established within the programme in each of the co-operating countries are called as PIACs (Principal International Alert Centres), being in close contact with the national pollution control structures. To meet the requirements on fast information transmission capabilities, the system are equipped with a satellite communication system linked to local computer network between the units of the PIACs with information processing system, hazardous substances database, the Danube Basin Alarm Model for the simulation of pollution effects. International Operation Manual provides the standard operation technique in each country. Practical example of the serious chemical accidental spill in Hungary in May 1998 illustrates the operation of the DAEWS system.