The observation of planetary atmospheres in the ultraviolet range provides specific advantages. A number of strong emissions from atoms and diatomic molecules appear in this spectral region; also a number of molecules have both continuumlike and narrow-band absorptions. In addition, for most atmospheres, the UV radiation does not penetrate to as deep levels as those probed in the visible, infrared and radio ranges because the UV penetration is limited by continuum molecular absorption, by absorption and scattering by small particles, and by rayleigh scattering. (The latter is particularly important for the thick hydrogen atmospheres of the outer planets.) Thus, the UV spectra are formed in the upper portions of a planet’s atmosphere and the IUE is an excellent tool for studying the upper atmospheres of planets, their aeronomy and their photochemistry. It is possible to divide the ultraviolet spectra of the planets into two types. Those associated with the region above the homopause (at roughly densities of 1012 cm-2) are of emissions from atoms and simple diatomic molecules in diffusive equilibrium above the more complex UV-absorbing molecules which are heavier and hence have smaller scale heights. The situation is illustrated in Figure 1 which shows altitude profiles of hydrocarbon mixing ratios for Jupiter computed by Gladstone (1982). H2 is the dominant species with a mixing ratio near 1.