In many ways marine and hydrokinetic power systems offer one of the most promising areas for renewable energy development. Marine and hydrokinetic resources – including ocean waves, tidal streams, river flows, ocean currents, and ocean thermal differences – are abundant, the technology is improving rapidly, and countries that have strong marine resources, such as the United Kingdom, United States, Australia, South Korea, Spain, and Portugal, have committed to supporting the industry. Marine energy, however, still faces significant technology, policy, and financing challenges.