A magnetic gas turbine sensor for sensing the speed and/or torque of a shaft in a gas turbine engine, the sensor comprising: a magnetic pole piece; a magnet associated with the pole piece; a conductive sensing element wrapped or wound around the pole piece and inductively coupled to the pole piece; and a first sensor casing surrounding and housing the pole piece, magnet and conductive sensing element; the first sensor casing defining a first inner fluid conduit for fluid coolant, the first fluid conduit being arranged to allow a fluid flow within the first sensor casing and running alongside and/or adjacent the pole piece, magnet and/or conductive sensing element, and wherein the sensor includes a first distal sensing end adjacent the pole piece for location adjacent the path of the shaft or an element rotating with the shaft, and a second proximal mounting end for mounting to a surface on or adjacent the engine characterised by the sensor also including a second outer sensor casing surrounding the first sensor casing and defining a second outer fluid conduit for fluid coolant, the outer fluid conduit being defined by the space between the first sensor casing and the second sensor casing and at least partially surrounding the first fluid conduit, and wherein the first fluid conduit has a fluid inlet towards the proximal mounting end of the sensor, the second fluid conduit has a fluid outlet towards the proximal mounting end of the sensor, and the first and second fluid conduits are in communication with each other in the sensor only in a region at the distal sensing end of the sensor and being arranged such that 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 to the outlet at the proximal mounting end. a magnetic pole piece; a magnet associated with the pole piece; a conductive sensing element wrapped or wound around the pole piece and inductively coupled to the pole piece; and a first sensor casing surrounding and housing the pole piece, magnet and conductive sensing element; the first sensor casing defining a first inner fluid conduit for fluid coolant, the first fluid conduit being arranged to allow a fluid flow within the first sensor casing and running alongside and/or adjacent the pole piece, magnet and/or conductive sensing element, and wherein the sensor includes a first distal sensing end adjacent the pole piece for location adjacent the path of the shaft or an element rotating with the shaft, and a second proximal mounting end for mounting to a surface on or adjacent the engine characterised by the sensor also including a second outer sensor casing surrounding the first sensor casing and defining a second outer fluid conduit for fluid coolant, the outer fluid conduit being defined by the space between the first sensor casing and the second sensor casing and at least partially surrounding the first fluid conduit, and wherein the first fluid conduit has a fluid inlet towards the proximal mounting end of the sensor, the second fluid conduit has a fluid outlet towards the proximal mounting end of the sensor, and the first and second fluid conduits are in communication with each other in the sensor only in a region at the distal sensing end of the sensor and being arranged such that 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 to the outlet at the proximal mounting end.