Operational Oceanography in Greece has become today a necessity, since it can provide the means and the solutions on societal, economic, environmental and scientific problems related with the coastal environment. A rational approach to these problems can be based on integrated coastal zone management, which, in turn, is supported by the systematic and continuous operation of marine monitoring systems. A real-time monitoring and forecasting system (called the ''POSEIDON'' system) is currently in operation in the Aegean Sea. The three main components of the POSEIDON system are: (a) an integrated marine monitoring network consisting of oceanographic and wave buoys (POSEIDON network), (b) a telecommunication system for real-time data transmission and (c) the Aegean Operational Forecasting System (AOFOS) including a meteorological model, an offshore wave model, a general circulation ocean model, a surface pollutant dispersion model and a shallow water wave model. Three years of experience has shown that the POSEIDON system is an essential and necessary tool for developing Operational Oceanography in Greece, being also the only integrated and multipurpose system of this nature in the whole Mediterranean Sea.