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The tidal flow of sea water induced by planetary motion is a potential source of energy if suitable systems can be designed and operated in a cost-effective manner. This paper examines the physical origins of the tides and how the local currents are influenced by the depth of the seabed and presence of land mass and associated coastal features. The available methods of extracting energy from tidal movement are classified into devices that store and release potential energy and those that capture kinetic energy directly. A survey is made of candidate designs and, for the most promising, the likely efficiency of energy conversion and methods of installing them are considered. Overall, the need to reduce CO2 emissions, a likely continued rise in fossil fuel cost will result in a significantly increased use of tidal energy. What is still required, especially for kinetic energy devices, is a much greater understanding of how they can be designed to withstand long-term immersion in the marine environment. [ABSTRACT FROM AUTHOR]