The motivation of this work is to develop high reflectance normal-incidence multilayer mirrors in the 8-12 nm wavelength region for applications in astronomy and extreme ultraviolet lithography. To achieve this goal, Mo/Sr and Mo/Y multilayers were studied. These multilayers were deposited with a UHV magnetron sputtering system and their reflectances were measured with synchrotron radiation. High normal-incidence reflectances of 23% at 8.8 nm, 40.8% at 9.4 nm, and 48.3% at 10.5 nm were achieved. However, the reflectance of Mo/Sr multilayers decreased rapidly after exposure to air. Attempts to use thin layers of carbon to passivate the surface of Mo/Sr multilayers were unsuccessful. Experimental results on the refractive index β δ i + = -1 ñ of yttrium and molybdenum in the 50-1300 eV energy region are reported in this work. This is the first time ever that values on the refractive index of yttrium are measured in this energy range. The absorption part β was determined through transmittance measurements. The dispersive part δ was calculated by means of the Kramers-Kronig formalism. The newly determined values of the refractive index of molybdenum are in excellent agreement with the published data. Those of yttrium are more accurate and contain fine structures around the yttrium M-absorption edges where Mo/Y multilayers operate. These improved sets of optical data lead to better design and modeling of the optical properties of Mo/Y multilayers.The reflectance quality of Mo/Y multilayers is dependent on their optical and structural properties. To correlate these properties with the multilayer reflectance, x-ray diffraction, Rutherford backscattering spectrometry, and transmission electron miscroscopy were used to Special thanks are due to Eric Gullikson for being an all around good guy at the ALS beamline and for his assistance in many of my measurements especially the grating studies. I would also like to thank Eric, Eberhard Spiller, Chris Walton, and Daniel Stearns for many enlightening discussions of my experimental data. I would like to thank John Seely for the grating efficiency calculations, Sherry Baker for the AFM measurements, Juergen Plitzko and Jennifer Harper for the TEM measurements and some discussion of the results, Cheng Saw for meaningful large-angle XRD measurements, iv Rick Gross for explaining about TEM sample preparation, and Dino Ciarlo for providing the silicon nitride membranes.I would like to thank the faculty at DAS (UC Davis) and EECS (UC Berkeley) for the graduate classes they taught me. I am grateful to Prof.