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.