In an immersion lithography apparatus, an isolator is provided between the substrate table and the projection system to, for example, prevent currents in the liquid exerting forces on the projection system that might tend to distort the reference frame to which said projection system is connected. The isolator may be maintained still relative to the reference frame by an actuator system responsive to a position sensor mounted on the reference frame. At least a portion of the isolator may have the same refractive index as the liquid.