Extreme ultraviolet light, with wavelengths between 1 nm and 100 nm, has promising applications in astrophysics, producing more compact integrated circuits, advanced imaging of nanometer-scale objects, and plasma diagnostics. Realizing this potential requires the design and production of optical elements which can efficiently reflect light at these wavelengths. One challenge faced in the design and production of these mirrors is correctly understanding and modeling the effects of surface irregularities on the nanometer length scale. Reasonable approximations for modeling the effects of roughness when it has a characteristic length scale much less than the wavelength of light or much greater than the wavelength of light. When the length scale is about the same size as a wavelength, more careful calculations are required. Our calculations have indicated that the commonly used approximations accounting for roughness are inadequate under many realistic conditions. This approach should also be applicable to the reflection of reflecting electromagnetic waves and acoustic waves from other rough surfaces with comparable geometries. I will outline an approach our group has implemented for these calculations and share some of the results we have seen to this point.