A lithographic apparatus has an illumination system that conditions a beam of radiation. A patterning support supports a patterning device, which serves to impart the beam of radiation with a pattern in its cross-section. A substrate support holds a substrate. A projection system projects the patterned beam of radiation onto a target portion of the substrate. A particle detection system detects a particle on a surface of an object. The particle detection system has a radiation source, which generates an illumination beam of radiation. The illumination beam of radiation is directed along a first optical path to a detection area at the surface of the object. A radiation detector receives a detection beam of radiation from the detection area along a second optical path. The length of the first optical path is made substantially equal to the length of the second optical path.