In recent years, the chemistry of dense cores has been revisited with the discovery of a plethora of multiply deuterated molecules and of large depletions of CO. The two processes are closely related, since the condensation of CO and other abundant species onto dust grains enhances deuterium fractionation in the gas phase by decreasing the destruction rate of H 3 + and its deuterated variants. Condensation of some stable species onto dust grains leads to systematic molecular differentiation, as is now firmly established in starless cores. The strong drop in abundance of CO and CS is naturally explained by depletion onto dust grains at densities above a few times 10 4 cm -3 . N 2 H + seems unaffected by this process up to densities of order a few times 10 5 – 10 6 cm -3 , while the NH 3 abundance may actually be enhanced in central regions of starless cores. Recent chemical calculations suggest that at densities in excess of 10 6 cm -3 , even the N-bearing species should eventually disappear from the gas phase. Under such "complete freezeout" conditions, H 3 + and its deuterated isotopologues become the best tracers of the molecular gas. As the density threshold for the complete freezout is time and model dependent, good observational constraints are needed. R ecent observations of the ground state submillimeter lines of deuterammonia suggest that this molecule is not completely frozen out in the high-density gas. In particular the submillimeter lines of ND 2 H are relatively strong, have moderate opacities and simple hyperfine patterns that allow accurate determination of the excitation temperature, H 2 volume density, and molecular column density. Observations of these lines thus provide new opportunities to study the physics and chemistry of cold, dense ISM, where most molecules are depleted onto dust grains. Planned absorption studies with the HIFI instrument on Herschel will also provide insights into the physics and chemistry of diffuse clouds along the lines of sight toward bright submillimeter continuum sources, where many complex molecules have already been detected.