Bearing failures are a major concern in wind turbine main gearboxes. The risk of bearing failure F(t) is calculated from the bearing life L. If this is done for all bearings in a gearbox, then, the bearing subsystem reliability for the required life R(H_req) = 1–F(H_req) may be calculated. Methods used for such calculations for wind gearboxes are well established and have been widely used in other fields.