Abstract: A lithographic apparatus is arranged to project a patterned radiation beam from a patterning device onto a substrate using a projection system. The lithographic apparatus comprises: an illumination system configured to condition a radiation beam; a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; a projection system configured to project the patterned radiation beam onto a target portion of the substrate; and a measurement system for measuring the wavefront aberration or other property of the apparatus. The measurement system comprises: a source module at the level of the substrate table for providing an effective source of radiation; and a sensor unit at the level of the support, for receiving radiation from the source module through the projection system for performing the measurement.