Millimeter and infrared observations of deuterated molecules

This contribution reviews the observations of deuterated molecules in the millimeter to far infrared wavelengths. It starts by reviewing the few observations in the literature of the two major reservoirs of deuterium, namely HD and atomic D, from which estimates of the elemental D/H ratio can be obtained. Then follows a review of the observations of less abundant molecules, where large degrees of deuteration (⩾1%) have been observed for almost 30 years. Singly deuterated molecules were the first to be observed and studied. More recently, doubly deuterated formaldehyde and ammonia have been discovered to be extremely abundant (⩾1% with respect to H2CO and NH3, respectively), much more abundant than the singly deuterated species squared and any present model would predict. The importance of these observations is described in length, both in terms of the comprehension of the mechanisms leading to the observed deuteration and of the possible applications of the theory. Particular emphasis is given to the most recent observations of doubly deuterated molecules and to the many questions that they arise.