The design of wind turbines developed from a background of work on propellers, airplanes, and helicopters. Computer codes developed for analyzing aerodynamics, forces, and vibration have been modified to examine wind turbines. Theory and experimental procedures are well developed, and no scientific breakthroughs are needed. However, there are problems of predicting loads from unsteady aerodynamics. These loads lead to material fatigue and less life than predicted by the design codes. Part of the time, wind turbine blades operate in regions of large attack angles that are very different from the stresses imposed on airplane wings.