An experimental and theoretical study of horizontal-axis wind turbines is undertaken. The theoretical analyses cover the four major areas of aerodynamics, turbulence. aeroelasticity and blade optimisation. EXisting aerodynamic theories based on blade-element theory for predicting the loads on a wind turbine blade are reviewed and extended to include non-uniform flow, tip losses and the 'turbulent wake' state. A theoretical model based on a free vortex representation of a turbine wake is developed enabling the flow field within the vicinity of a turbine to be ascertained and also providing an important cross-check with the far simpler blade~lement theory. The turbulent variations in the wind velocity over the rotor swept area are considered and in particular their effects on the loads and available energy. A simple numerical solution of the dynamics of a two-bladed rotor is developed enabling the loads within the rotor blade to be determined and also those which are transferred to the supporting structure. Three rotor configurations are examined in detail; hingeless, hinged and teetered. The important problem of determining the appropriate blade shape for a particular application is considered. In parallel with the theoretical study an experimental program is also undertaken. This includes the design, construction and testing of three turbines. The design of these turbines embodies the results from 2 the theoretical studies. Wherever possible the experimental results derived from measurements made on the three experimental turbines are used to test the accuracy of the theoretical analyses.