The interdisciplinary and international project BASYS (Baltic Sea System Studies) was financed by the Europeen Union in the years 1996 to 1999 and investigated many aspects and influences of mankind activities on the ecosystem Baltic Sea as well as the natural influences such as climate and weather. A large database accessible to all scientists was collected during the project and should help in the future to distinguish between the natural and human effects upon the ecosystem of the Baltic Sea. This is important for the management of the damaged ecosystem and to insure that in future a permanent steady state can be reestablished in the Baltic Sea.