POSEIDON system is an infrastructure for monitoring, forecasting and information system for the Greek seas, being at the leading edge of modern oceanography in Europe. The network of observation buoys records continuously the physical, biological and chemical parameters of the Greek seas. Those data are then transmitted to the operational centre where they are sorted and fed into forecasting models. POSEIDON system is a unique planning tool in the endeavour for the protection of the marine environment. It also provides a competitive advantage for the development of business activity, the prevention of disaster, and the safeguarding of human life. In the frame of the world-wide trend for the development of operational oceanography, POSEIDON network places Greece among the leading countries in this field.