The invention relates to a projection objective (6), in particular for applications in microlithography, serving to project an image of an object (3) arranged in an object plane (4) onto a substrate (18) arranged in an image plane (7). The projection objective (6) has an object-side-oriented part (10) which is arranged adjacent to the object plane (4) and includes a plurality of optical elements, and it also has an image-side-oriented part (11) of the objective which is arranged adjacent to the image plane (7) and includes a free space (16) serving to receive a fluid (13) and further includes at least a part of an optical end-position element (14) serving to delimit the free space (16) towards the object side. The projection objective (6) is operable in different modes of operation in which the free space (16) is filled with fluids (13) that differ in their respective indices of refraction.