Chapter 12 introduces offshore wind power, beginning with a historical overview from the first offshore wind turbine installed in 1990, to the gigawatt-scale arrays now under development. The technologogical progress of both fixed and floating arrays is described. Offshore wind characterisics are discussed and metocean data (including the influences of wind, wave, and currents) described. Wind conditions on- and offshore are compared. Wave characteristics are described in detail, based on linear wave theory and Jonswap spectral characteristics, with simple equations given for significant and maximum wave height, and wave and current velocity profiles. The combined forces on an offshore turbine stuctures are described with a worked example of wind, wave, and current loading on a monopile. Aspects of offshore wind turbine design are discussed, including marinisation, the influence of blade size, and drivetrain architecture, and a short section describes modern offshore installation vessels. The historic performance of offshore arrays is assessed with data for the UK and Danish sectors, and the chapter concludes with a review of environmental impact issues for offshore developments.