A lithographic apparatus that includes an illumination system configured to condition a radiation beam. The illumination system includes a plurality of optical components. The apparatus also includes a support constructed to support a patterning device. The patterning device is capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam. The apparatus further includes a substrate table constructed to hold a substrate, and a projection system configured to project the patterned radiation beam onto a target portion of the substrate. The projection system includes a plurality of optical components. The apparatus also includes a contamination measurement unit for measuring contamination of a surface of at least one of the optical components. The contamination measurement unit is provided with a radiation sensor constructed and arranged to measure an optical characteristic of radiation received from the surface.