The on-going cost and difficulty in maintaining a reliable supply of fossil or biomass fuel make gen sets impractical for some off-grid applications. Wind turbines, hydro turbines, and photovoltaic arrays have no fuel costs. Instead, they make use of locally available and replenishable energy sources. This chapter describes the principles of operation of wind energy conversion systems (WECSs), introducing topics such as tip speed ratio, power coefficient, and power curves. A circuit model is derived. Particular attention is given to the use of WECS for battery charging and modeling of the wind resource. Economic, environmental, and social considerations of the use of wind energy for off-grid electrification are discussed.