Ocean Energy belongs among renewable energies certainly to the group of hydro based energy sources. Conditioned by its generating factors, it can be mainly subdivided into Osmotic, Tidal, Tidal Current, Thermal and Wave energy. In Osmotic Energy, the process of water desalination is responsible for the energy extraction by harnessing the osmotic pressure. Tidal Energy results from the influence of the moon's gravitation on the earth's oceans, and is classically performed over the combination of hydraulic dams and turbines. Tidal Current Energy relies on the same physical principle, but uses the kinetic energy of the flow instead of the head in to produce electricity. In Thermal Energy, the temperature difference between the ocean surface and the temperature in the deeper sea grounds exploits the energy from the seas. Wave energy is produced by the wind generated over the solar heating of the ocean's surface. It is last group that presents the greatest variety of application devices, depending on its working principle. Mainly it can be subdivided into Terminators, Attenuators and Point Absorbers.

Historically, Ocean Energy Devices are being introduced by tidal mulls in the15th century in several churches around the coast of England, used to trigger meal. The first world wide invention belongs to "Gerard et fils" by a patented tidal pump in 1799. The first patented wave energy device to deliver electricity into the grid is a sea-snake similar structure invented by Cockerell in 1978. A total of more than 1000 patents have been presented in the last half of the 20th century solely concerning to wave energy. However, the most important ocean energy project validated until the date is the tidal power plant in la Rance, France. Installed in 1967 with a capacity of around 60 MW, it has been working since then.

More generally, Tidal and Wave Energy seem to be the categories with better commercialization perspectives. This is clearly shown by the fact, that from nearly 100 institutions & companies worldwide related to ocean energy, almost 50 % focuse on wave energy. Nevertheless, research is increasing definitely in the other fields, too. Companies like Voith Siemens, Npower Renewables or Biopower Systems are e.g. involved in several fields focusing mainly on the wave & tidal sectors. To this fact it is important to note that Europe is the leading continent in terms of research & development. Apart from the examples mentioned above this is due to positive government conditions for the use of new renewable conditions. The new renewable Energy Law in Germany (2000) e.g. or the Marine Supply Obligation (2006) are clear examples of how to promote the installation of Ocean Energy. Still, governments have to work narrower, and companies have to find solutions for issues such as the grid connection.

So, the commercial development of Ocean Energy plants has increased, too. In Mutriku, Spain, the first commercial breakwater turbine plant with about 0.4 MW is being installed by Voith Siemens Hydro. The same company plans the installation over its joint venture company Renetec of tidal current turbines in the South Korean coast with a rated power of nearly 600 MW. Another plant with about 15-20 MW based on the Oscillating Water Column technology is going to get installed off the United States by Oceanlinx. Finnavera Renewables have gotten the permission to start the production of several AQUABUOY point absorbers devices, planned offshore from the Californian Coast with a capacity of 1 MW. Another future commercialized point absorber device project is the SEAREV project, with nearly 500 kW rated power output. Last but not least, a 1.3 MW tidal current device called SEAGEN has been installed and connected to the grid in April 2008 off the Northern Irish Coast.