The application relates to an apparatus (100) for fatigue testing a wind turbine blade, and to a system and method using such an apparatus (100). The apparatus (100) comprises a base (110) for supporting a first end (12) of the wind turbine blade (10), and an edgewise actuator assembly (120). The edgewise actuator assembly (120) includes a ground-supported edgewise actuator (130) and a flexible cable assembly (140) for connecting the edgewise actuator (130) to the blade (10). The edgewise actuator (130) and the flexible cable assembly (140) are adapted to cyclically deflect the blade (10) relative to the base in the edgewise direction by repeatedly pulling the blade (10) in a substantially horizontal direction.