Abstract: An illumination system for a microlithography projection exposure apparatus is designed for illuminating an illumination field with an illumination radiation with a predeterminable degree of coherence ?, it being possible to adjust the degree of coherence within a degree of coherence range extending into the range of very small degrees of coherence of significantly less than ?=0.2. The illumination system may have a first optical system for generating a predeterminable light distribution in an entrance plane of a light mixing device, and also a light mixing device for homogenizing the impinging radiation. The first optical system and the light mixing device can in each case be changed over between a plurality of configurations corresponding to different degree of coherence ranges. The degree of coherence ranges overlap and are dimensioned such that the resulting total degree of coherence range is larger than the individual degree of coherence ranges.