When viewed from above the Earth's atmosphere, the nighttime ultraviolet sky background is profoundly dark. Recent measurements indicate that the diffuse UV sky background is up to 100 times (5 magnitudes) fainter than the equivalent visible background as measured from the ground. Much of this difference can be attributed to the Sun's lower emissivity at UV wavelengths, leading to reduced irradiation of and scattering by the interplanetary dust. Because the resulting Zodiacal light is so much weaker in the UV, a comprehensive characterization of the UV sky can yield important information on the more distant Galactic and extragalactic backgrounds and, ultimately, on their material origins (see Brosch, this volume, p. 57).