The effective management and protection of the marine ecosystem as requested by European latest directives can be achieved only by an operational monitoring and forecasting of marine environmental conditions that require the use of multivariable real-time measurements combined with advanced physical and ecological numerical models. A modern approach to the ocean observations involves a variety of sources, including satellite-based instruments, in-situ platforms such as surface and sub-surface buoys and floats, autonomous vehicles and volunteer observing ships. Fixed oceanographic buoys still remain among the more powerful and cost-effective tool for collecting data over the oceans and they contribute significantly to operational monitoring especially when continuous, long-term, high frequency and multiple parameters are among the requirements. The Mediterranean Moored Multi-sensor Array network of open ocean observing systems operates by the end of the '90ies as primary providers of near real-time in-situ observations incorporating state-of-the-art sensor for optical and chemical measurements in the surface layers of the ocean and physical measurements in the ocean interior down to the seafloor. The network is composed by three independent observatories deployed in the Ligurian sea, the south Adriatic sea and the Cretan sea. Although the three observatories had different past objectives, they are managed by different Institutions and they are instrument to answer to site-specific scientific issues, they have been upgraded in the recent years to a common conceptual structure. Furthermore, they are now characterized by common components such as sampling strategies, similar or partly interchangeable instrumentation, sharing of calibration facilities, data management architectures and quality control procedures. The network is part of national, European and international alliances to respond to local and global needs in ocean monitoring.