The vapour pressures of ice and supercooled water are reviewed with an emphasis on atmospheric applications. Parametrizations are given for the vapour pressure, molar heat capacity, and latent heat of vaporization of both ice and liquid water. For ice, the experimental vapour pressure data are in agreement with a derivation from the Clapeyron equation. Below 200 K cubic ice may affect the vapour pressure of ice both in the atmosphere and in the laboratory. All of the commonly used parametrizations for the vapour pressure of supercooled water are extrapolations that were not originally intended for use below the freezing point. In addition, the World Meteorological Organization definition of the vapour pressure of supercooled water contains an easily overlooked typographical error. Recent data on the molar heat capacity of supercooled water are used to derive its vapour pressure. Nevertheless, the uncertainty is such that measurements of the deliquescence and freezing behaviour of aerosol particles are beginning to be limited by uncertainties in the thermodynamics of supercooled water. Copyright © 2005 Royal Meteorological Society