The extreme ultraviolet reflectance of aluminum prepared under optimized conditions is reviewed, including a study of the aging parameters. The theory of reflectance-increasing films for the vacuum ultraviolet is discussed. It is shown that single dielectric films, in addition to preventing the growth of an oxide film, can have a surprisingly strong reflectance-increasing effect on aluminum in this spectral region. The use of MgF2 as a reflectance-increasing coating for the extreme ultraviolet above 1100 A is considered in some detail. It is pointed out that two-layer reflectance-increasing coatings on aluminum have only a very small advantage over single-layer coatings in the extreme ultraviolet. The usefulness of single slightly absorbing films as reflectance-increasing coatings is treated. The problem of increasing reflectance in the spectral region below 800 A is discussed. A new apparatus is described which allows the preparation and measurement of film samples without exposure to air. The effect of the new high-reflectance coatings on the relative merit of vacuum ultraviolet monochromator designs is considered.