The at-wavelength interferometry of projection optics,which has important research significance and practical value in integration of lithography tool,is one of the core enabling techniques for extreme ultraviolet lithography (EUVL) facing 32 nm technology node and beyond. In this paper,recent developments of at-wavelength interferometry of EUVL optics are reviewed. Several key interferometry techniques are presented in details. The important parameters of the key techniques,such as,measurement accuracy,speed and dynamic range etc. are analyzed and compared. The core techniques in developing at-wavelength intereferometry device are analyzed. Finally,a summary is given and some prospects for at-wavelength interferometry of EUVL optics are proposed.