In a projection objective provided for imaging a pattern arranged in an object plane of the projection objective into an image plane of the projection objective with the aid of an immersion medium arranged between a last optical element of the projection objective in the light path and the image plane, the last optical element has a transparent substrate and a protective layer system that is fitted to the substrate, is provided for contact with the immersion medium and serves for increasing the resistance of the last optical element to degradation caused by the immersion medium.