A gravity gradiometer is disclosed which has a sensor in the form of bars (41 and 42) which are supported on a mounting (5) which has a first mount section (10) and a second mount section (20). A first flexure web (33) pivotally couples the first and second mount sections about a first axis. The second mount has a first part (25), a second part (26) and a third part (27). The parts (25 and 26) are connected by a second flexure web (37) and the parts (26 and 27) are connected by a third flexure web (35). The bars (41 and 42) are located in housings (45 and 47) and form a monolithic structure with the housings (45 and 47) respectively. The housings (45 and 47) are connected to opposite sides of the second mount section 20. The bars (41 and 42) are connected to their respective housings by flexure webs (59). Transducers (71) are located in proximity to the bars for detecting movement of the bars to in turn enable the gravitational gradient tensor to be measured. An actuator is provided for moving the mount to stabilize the mount. The actuator comprises a hollow disc housing (310) which defines a chamber (312). A coil plate (313) is located in the chamber (312) and supports windings (W1 and W2). A tube (330) supports a hollow rod (328) to minimize flux leakage from the windings (W1 and W2). Lead wires for the windings pass through the rod (328) to the windings (W1 and W2) mounted on the support plate (313).