Position and curvature information of a patterning device may be determined directly from the patterning device and controlled based on the determined information. In an embodiment, a lithographic apparatus includes a position determining system operative to determine a relative position of the patterning device. The patterning device may be configured to create a patterned radiation beam from a radiation beam incident on a major surface of the patterning device. The patterning device may have a side surface having an edge in common with the major surface. The position determining system may include an interferometer operative to transmit light to the side surface and to receive the transmitted light after the transmitted light has been reflected at the side surface. The position determining system is operative to determine a quantity representative of the relative position of the patterning device from the received reflected transmitted light.