An apparatus (10) for microlithographic projection exposure, which includes: an optical system (18) for imaging mask structures (16) onto a surface (21) of a substrate (20) by projecting the mask structures (16) with imaging radiation (13), the optical system (18) being configured to operate in the EUV and/or higher frequency wavelength range, and various structure defining a measurement beam path (36) for guiding measurement radiation (34), the measurement beam path (36) extending within the optical system (18) such that the measurement radiation (34) only partially passes through the optical system (18) during operation of the apparatus (10).