A lithographic apparatus having a projection system that includes a plurality of mirrors arranged to project the patterned radiation beam onto a target portion of the substrate. The lithographic apparatus further includes an autofocus system having a light source, a position sensitive detector, and a light directing arrangement for directing a first portion of light from the light source to the position sensitive detector. The light directing arrangement further directs a second portion of light from the light source into the projection system and, after the second portion of light has traveled through the projection system to the substrate and back through the projection system to the light directing arrangement, the second portion of light is directed onto the position sensitive detector.