Diffraction-limited optical imaging systems for extreme ultraviolet (EUV) lithography require wave front aberrations to be limited to a fraction of the EUV wavelength. Surface figure metrology and wave front measurement at this level : of accuracy represent key challenges in the development of EUV lithography. We have constructed and operated a wave front measuring interferometer at 12-13 nm wavelength. This interferometer is being used to measure the aberrations in 0.1 numerical aperture Fresnel zone plate lenses. Factors limiting the resolution and accuracy of the interferometer were studied. Substantial progress toward lambda/20 wave front measurement accuracy has been made.