1. Introduction: How can continuous and reliable monitoring at the seafloor and along the water column by means of Seafloor Observatories extend the exploration and improve knowledge of our Planet?- Section 1 - Present scientific challenges to be addressed with seafloor observatories 2. Integrating continuous observatory data from the coast to the abyss: Assembling a multidisciplinary view of the ocean in four dimensions,- 3. Underwater laboratories for Astro-Particle Physics and deep-sea physics, 4. Seafloor observations and observatory activities in the Sea of Marmara,- 5. The Hellenic deep sea observatory. Science objectives and implementation, 6. Marine seismogenic-tsunamigenic prone areas: the Gulf of Cadiz,- Section 2- Technical solutions for seafloor observatory architecture,- 7. The role of Information Communication Technologies (ICT) for seafloor observatories: acquisition, archival, analysis, interoperability,- 8. Long-term sub-sea observatories: comparison of architectures and solutions for infrastrucure design, interfaces, materials,sensor protection and deployment operations,- 9. Development and demonstration of a mobile response observatory prototype for sub-sea environmental monitoring: the case of ROSE,- 10. Construction of the DONET real-time seafloor observatory for earthquakes and tsunami monitoring,- 11. GEOSTAR-class observatories 1995-2012 - a technical overview,- Section 3- World-wide recent and ongoing projects and programmes 12. The two seafloor geomagnetic observatories operating in the western Pacific,- 13. The DELOS project; devlopment of a long-term observatory in an oil field environment inthe Tropical Atlantic Ocean,- 14. Sub-sea environmental observatory integrated with the KM3NET neutrino telescope infrastructure in the Mediterranean Sea,- 15. ANTARES Neutrino telescope and deep-sea observatory,-16. The NEPTUNE Canada Project: installation and initial operation of the world’s first regional cabled ocean observatory,- 17. The ALOHA cabled observatory,- 18. Next-generation science in the ocean basins: Expanding the oceanographer's tool-box utilizing submarine electro-optical sensor networks,- 19 Technical preparation of long-term cabled observation from Chinese marginal seas,- 20. From ESONET multidisciplinary scientific community to EMSO novel Euorpean research infrastructure for ocean observation,- Section 4 - Relevant scientific results with a multidisciplinary emphasis - 21. Seafloor observatory for monitoring hydrological and geological phenomena associated with seismogenic subduction zones,- 22. Modelling of regional geomagnetic field based on ground observation network including seafloor geomagnetic observatories,- 23. Seafloor borehole observatories in Northwestern Pacific,- 24. A first insight into the Marsili volcanic seamount (Tyrrhenian Sea, Italy), results from ORION-GEOSTAR3 experiment,- 25. Development and application of advanced ocean floor network system for megathrust earthquakes and tsunamis ,- Concluding remarks - 26. Perspectives and long-term vision.