Deuterium is particularly interesting because stellar processing and the recycling of gas through stars (astration) generally cause it to be destroyed and not created, and its very existence is an argument for the Hot Big Bang as was originally stressed by Gamow at a time when its presence had only been established in terrestrial and meteoritic water where it is enhanced by a factor of about 6 owing to fractionation. That this is so was established from studies of the solar wind in meteorites and lunar foils and soils from 1970 onwards (Black 1971, 1972; Geiss & Reeves 1972; Boesgaard & Steigman 1985). The Solar wind contains 3He inherited from the interstellar medium (ISM) when the Solar System was formed 4.6 Gyr ago; 3He resulting from destruction of proto-solar deuterium; and (perhaps) 3He dredged up by turbulent mixing from nuclear-processed material deep inside the Sun (Schatzman 1987). The result is 3He / 4He = (4.0 ± 0.2 (s.e.)) × 10-4 or, assuming He/H = 0.1 in the Sun, y23 3He / H = (4.0 ± 0.2) × 10-5. On the other hand, gas released by heating carbonaceous chondrites, believed to represent solar wind particles implanted near the time of the birth of the Solar System, contains a smaller proportion of 3He, 3He / 4He = (1.52 ± .05) × 10-4, corresponding to the proto-solar abundance y3 of 3He on its own. If fresh production of 3He by dredge-up is ignored, the proto-solar D / H ratio is simply the difference y23 - y3 = (2.5±0.2) × 10-5, with which ground-based and Voyager infrared observations of deuterated molecules (HD, CH3D) in the atmospheres of Jupiter, Saturn and Uranus, carried out since 1972, are in fair agreement (see Boesgaard & Steigman 1985; Pagel 1987a). Interstellar deuterium was discovered in 1973, in molecular form from radio observations of molecular clouds (Jefferts, Penzias & Wilson 1973) and as HD and DI (Lyman bands and Lyman series) from ultra-violet spectroscopy of diffuse clouds in front of hot stars using the Copernicus satellite (Spitzer et al. 1973; Rogerson & York 1973). A relatively low abundance of DCO+ and DCN at the Galactic centre (Penzias 1979) supports the purely (or at least mainly) destructive effect of astration on deuterium, but unknown fractionation effects make it difficult to infer the interstellar D / H ratio from molecules except in the case of DCO+ (Dalgarno & Lepp 1984) which agrees with atomic lines in giving a ratio ~ 10-5. A hyperfine transition of DI, at 91.6 cm wavelength, has been searched for several times, but without a definite detection (Pasachoff & Vidal-Madjar 1989). Most determinations of the interstellar D/H ratio come from observations of Ly , , in absorption on lines of sight to hot stars at distances up to 1 kpc and a few from IUE observations of Ly emission lines, with interstellar absorption superposed, from very nearby stars, the deuterium line appearing as a weak component displaced to the violet by 81 km s-1. Difficulties arise from appropriate modelling of the velocities and velocity dispersions of the intervening clouds (done with the help of optical observations of NaI) and from the possibility of spurious signals arising from hydrogen clouds expelled at about 80 km s-1 from the target star, for which there is direct evidence in some cases (Vidal-Madjar et al. 1983; Gry, Lamers & Vidal-Madjar 1984). Estimates of the interstellar D / H ratio thus cover quite a wide range, from 2.5 × 10-5, the same as the proto-solar value, to 6 × 10-6 (Boesgaard & Steigman 1985; Pasachoff & Vidal-Madjar 1989).