A lithographic apparatus comprises a substrate table for holding a substrate and a projection system for projecting a radiation beam onto a target region of the substrate so as to form an image on the substrate. The projection system comprises a lens element arrangement having a first lens element. A first pressure sensor is arranged to measure at least one pressure value adjacent the first lens element. A controller determines a first change in a pressure difference over the first lens element and/or a further lens element based on a signal received from the pressure sensor, determines adjustments to a position of one of the substrate table and projection system based upon the determined first change, and causes actuators to make adjustments to the substrate table or the projection system.