A bi-directional fluid flow sensor is disclosed. The sensor comprises a turbine configured to rotate along an axis as a result of fluid flowing past the turbine substantially in the direction of the axis; a magnet mounted to the turbine and configured to rotate with the turbine; and a magnetic flux angle sensor provided adjacent to the turbine and configured to detect the relative flux angle of the magnet as it rotates. The bi-directional fluid flow sensor is sensitive and robust and may be used in a downhole well for example.