An inspection apparatus for measuring a property of a substrate (W), includes a base frame, a substrate table (51) to hold the substrate, an illumination system arranged to direct a beam of radiation onto the substrate and a sensor arranged to detect radiation reflected off the substrate. Two balanced positioning systems displace the substrate table and sensor relative to the base frame in several directions. Each balanced positioning system includes a balance mass (59, 61), a bearing arrangement (65) to movably support the balance mass and tracks effective to guide the displacement in each direction. A motor arrangement causes the displacement in each direction. The balance mass is positioned relative to the track arrangement such that the centers of gravity of each balance mass and the substrate table or the sensor are substantially aligned in the direction substantially perpendicular to the plane including the direction of displacement.