In this chapter, Ocean Thermal Energy Harvesting (OTEC), which is an energy-generating technology that takes advantage of the temperature difference between the ocean's shallow warm water and cold deeper water, is investigated. It discusses the closed, open and hybrid-cycle OTEC systems as well as their required components. Closed-cycle OTEC process was first proposed in 1881 by French physicist Jacques D'Arsonval. Closed-cycle OTECs use a working fluid with a low boiling point, such as ammonia, to rotate a turbine of heat engine to generate electricity. In contrary to the closed-cycle OTEC systems, seawater is used as the effective working fluid in the open-cycle OTEC systems. In 1993, the largest open-cycle OTEC plant was designed by Pacific International Center for High Technology Research. In a hybrid OTEC system, warm seawater first enters a vacuum chamber, where it is partly flash-evaporated into steam, similar to the open-cycle evaporation process.