Today front-surface mirrors are used extensively in optical devices from the extreme ultraviolet to the far infrared. In this paper, methods for measuring the reflectance of mirror coatings at various wavelength regions, and techniques for preparing reflecting films of maximum reflectance, adherence, and durability will be described. Data on the ultraviolet, visible, and infrared reflectance of the most frequently used evaporated mirror coatings, Al, Au, Ag, Cu, and Rh will be presented. It will be shown that Al is the only film material that has high reflectance in all spectral regions. It also adheres better to glass and other substrates than the other reflecting coatings, does not tarnish in normal air, and is easy to evaporate. This makes it the most useful mirror coating. The application of single and multilayer protective layers for increasing the durability and increasing the reflectance of mirror coatings will be described and their effect on the reflectance at close to normal and higher angles of incidence will be treated. It will be shown that Al and Ag protected with rather thin silicon oxide or Al2O3 films (1000–2000 Å) have a normal incidence reflectance which is practically identical with that of unprotected mirrors in the infrared from 8 to 12 μm, but that their reflectances at higher angles of incidence is greatly decreased. This decrease is caused by the decrease in Rp rather than Rs, resulting in a sensitivity to polarization that makes possible a very efficient polarizer for the CO2 laser (10.6 μm) using four mirrors coated with Al+Al2O3.