The viscosity of a liquid measures its resistance to flow, with consequences for hydraulic machinery, locomotion of microorganisms, and flow of blood in vessels and sap in trees. Viscosity increases dramatically upon cooling, until dynamical arrest when a glassy state is reached. Water is a notoriously poor glassformer, and the supercooled liquid crystallizes easily, making the measurement of its viscosity a challenging task. Here we report viscosity of water supercooled close to the limit of homogeneous crystallization. Our values contradict earlier data. A single power law reproduces the 50-fold variation of viscosity up to the boiling point. Our results allow us to test the Stokes-Einstein and Stokes-Einstein-Debye relations that link viscosity, a macroscopic property, to the molecular translational and rotational diffusion, respectively. In molecular glassformers or liquid metals, the violation of the Stokes-Einstein relation signals the onset of spatially heterogeneous dynamics and collective motions. Although the viscosity of water strongly decouples from translational motion, a scaling with rotational motion remains, similar to canonical glassformers.supercooled water | viscosity | Stokes-Einstein relations W ater, considered as a potential glassformer, has been a longlasting topic of intense activity. Its possible liquid-glass transition was reported 50 years ago to be in the vicinity of 140 K (1, 2). However, ice nucleation hinders the access to this transition from the liquid side. Bypassing crystallization requires hyperquenching the liquid at tremendous cooling rates, ca. 10 7 K · s −1 (3). As a consequence, many questions about supercooled and glassy water and its glass-liquid transition remain open (4-7).As an example, crystallization of water is accompanied by one of the largest known relative changes in sound velocity, which has been attributed to the relaxation effects of the hydrogen bond network (8, 9). Indeed, whereas the sound velocity is around 1,400 m · s −1 in liquid water at 273 K, it reaches around 3,300 m · s −1 in ice at 273 K and a similar value in the known amorphous phases of ice at 80 K (10). Such a large jump is usually the signature of a strong glass, i.e., one in which relaxation times or viscosity follow an Arrhenius law upon cooling. However, pioneering measurements on bulk supercooled water by NMR (11) and quasi-elastic neutron scattering (12), as well as recent ones by optical Kerr effect (8, 9), reveal a large super-Arrhenius behavior between 340 and 240 K, similar to what is observed in fragile glassformers (13,14). The temperature dependence of the relaxation time is well described by a power law (8, 9), as expected from mode-coupling theory (15, 16), which usually applies well to liquids with a small change of sound velocity upon vitrification. Based on these and other observations, it has been hypothesized that supercooled water experiences a fragile-tostrong transition (17). This idea has motivated experimental efforts to measure dynamic properties of supercooled wate...