Marine hydrokinetic energy includes that due to waves, tides, and ocean currents. The characteristics of these forms of energy and the assessment of their potential for extraction are discussed briefly herein. Detailed consideration is given to the assessment of ocean current energy, including a case study of the resource assessment of the Florida Current. Estimates of global and local open ocean current resources are obtained based on data from an ocean model. The power densities of major western boundary currents are estimated and the potential for development of ocean currents globally is assessed. Principal factors that govern economic viability of harnessing an ocean current at a location include the in-situ power density, the distance of the location from the shore, and the local depth of the seafloor. A metrics based on these considerations is discussed. Once potential sites are identified, considerations would need to be given to regulatory and permitting requirements, including assessment of potential impact on the environment and its ecosystems, marine spatial planning and the impact on the energy resource itself; development of optimal design of devices for high performance; and mitigation of deployment and maintenance costs.