Fatigue testing of wind turbine blades provides extremely valuable data for both blade manufacturers and turbine end-users, in terms of design validation and certification for in-service requirements [1]. However, existing tests tend to use only one loading direction at a time – flap and edge testing done separately – which is a gross simplification of the fatigue experienced by blades in service. Fatigue testing of blades can be achieved using either forced actuation or resonance methods. Various different practical means for introducing the loads into the blade have been devised, including the use of hydraulic actuators for forced actuation and the use of rotating eccentric masses to induce resonant loading. A second method for inducing resonance using sliding masses mounted on the blade, powered by hydraulics, was pioneered by NREL (National Renewable Energy Laboratory) in the USA [2], and has been developed further by NaREC in the UK [3]. NaREC’s development work in hydraulically powered resonant methods has lead to a test method that now allows both flap and edge loading of blades to be achieved simultaneously. This is a valuable contribution to the wind industry’s desire to reduce testing costs, whilst improving the rigour of the testing methods.