The hydration dynamics of both DNA and RNA helices present common features. They are spatially very heterogeneous: while most of the hydration shell water molecules, in solvent-exposed sites, are only moderately slowed down compared to bulk water (τ reor < 10 ps, i.e. ρ reor < 6), water molecules located in the narrow grooves (DNA minor groove and RNA deep major groove) exhibit a dynamics up to 15-20 times slower than the one observed in the bulk. These findings are consistent, for DNA, with NMR MRD measurements 18 that evidenced a moderate 6-fold slowdown in most of the hydration shell, and the presence of a few water molecules in the grooves with longer ∼ 200 ps residence times. It contrasts with the bulk-like diffusion dynamics suggested by Overhauser effect dynamic nuclear polarization, which provides an estimate of the translational dynamics of water molecules within about 10 Å of the probe. The discrepancy between these recent experiments and other experimental and computational techniques 53 still remains to be explained, and could be related either to the impact of the probe on water dynamics, or to some assumptions used in the model to interpret the data. Interestingly, as noted in earlier computational studies, the location of the slowest water molecules differs between DNA and RNA sequences: in DNA, they are found at the floor of the groove, where they are H-bonded to DNA bases, whereas in RNA they lie on the side of the major groove, next to the phosphate groups.