Maintenance for wind turbines, particularly offshore turbines, presents a significant cost component to wind farm operators. The main bearing, which supports the lowspeed shaft, is one of the major wind turbine system components. Since maintenance is confined to suitable weather windows, which are unpredictable, there is a need for decision support tools to ensure that maintenance is carried out in a timely way, but also at minimum cost and with minimum turbine downtime. This paper presents a methodology for the estimation of the remaining useful life (RUL) of the main bearing for a commercial wind turbine. A residual model is used to highlight potentially faulty behaviour, which is then post processed to provide a suitable signal for extrapolation using particle filters. The RUL is then effectively specified as a probability distribution which narrows as the failure point is approached, providing an estimate of RUL and a confidence measure. Our results suggest that RULs beyond 30 days can be reasonably estimated.