An illumination system of a microlithographic projection exposure apparatus comprises a light source which is configured to produce projection light (PL), a pupil plane and a diffractive optical element that is arranged between the light source and the pupil plane such that an irradiance distribution of projection light (PL) in the pupil plane depends on the position of a field that is illuminated by the projection light (PL) on the diffractive optical element. The illumination system further comprises an optical imaging system that is arranged between the light source and the diffractive optical element. The optical imaging system ensures that changes of the direction and divergence of the projection light (PL) emitted by the light source have no substantial effect on the position and size of the field which is illuminated on the diffractive optical element by the projection light (PL).