During the past decade, vacuum ultraviolet spectra of over 30 comets have been obtained with the International Ultraviolet Explorer (IUE) satellite observatory. With few exceptions, the spectra of these comets appear to be similar, with OH and H produced by the photodissociation of water being the dominant species and emissions of C, O, S, CS and CO+
2 usually present. Although signs of variability of many kinds in comet spectra appear, the evidence from the ultraviolet observations suggests that all comets have the same basic chemical composition and that observed differences are due to evolution and ageing processes. The principal exception is S2, which was detected by IUE in comet IRAS-Araki-Alcock (1983 VII), but not in any other comet to date. During the 1985–86 apparition of comet Halley, ultraviolet spectra were also obtained by other spacecraft and by sounding rocket instruments, including a long-slit imaging spectrograph. Further advances await future ultraviolet observations of comets by the Hubble Space Telescope and other planned ultraviolet astronomy missions.