In many low-charge ions of astrophysically abundant light elements, the spins of the ground and first-excited terms are different. Because of spin-orbit interactions, the states of these terms are mixtures of LS basis states, and as a consequence, transitions with ∆S = 1, i.e., ‘intersystem transitions’, can occur. The_transition probabilities (A-values) for such lines in low-Z ions are about 104 sec -1 to 102 sec-1. These values are of the same order of magnitude as the collisional de-excitation rates in many low-density astronomical objects. Consequently, intersystem lines, which are not readily seen in the laboratory, are significant features in many astronomical spectra. IUE spectra that show such lines in novae, hot stars, cool stars, symbiotic stars, binary stars, variable stars, Herbig-Haro objects, H II regions, planetary nebulae, quasars, and galaxies are discussed in Kondo et al. (1982). Moreover, because of the commensurate radiative and collisional de-excitation rates for the upper levels, the ratios of intersystem-line intensities to allowed-line intensities are density sensitive in many objects.