A magnetic gas turbine sensor (25) for sensing the speed and/or torque of a shaft in a gas turbine engine, the sensor comprising a magnetically energisable pole piece (3), a magnet (2) associated with the pole piece and a conductive sensing element (4) wrapped or wound around the pole piece (3) and inductively coupled to the pole piece. The sensor includes a first sensor casing including a first inner fluid conduit (36) for fluid coolant, the first fluid conduit being inside the casing and running alongside and/or adjacent the pole piece, magnet and/or conductive sensing element, and the sensor also including a second sensor casing surrounding the first sensor casing and defining a second outer fluid conduit (37) for fluid coolant and at least partially surrounding the first fluid conduit. Fluid coolant may flow into the sensor at its proximal mounting end, through the first fluid conduit over or through the pole piece, magnet and/or conductive sensing element to the sensing end, and then through the second fluid conduit from the distal sensing end (29) to the outlet at the proximal mounting end (31).