  We review the general theoretical concepts and observational constraints on
the distribution and evolution of water vapor and ice in protoplanetary disks,
with a focus on the Solar System. Water is expected to freeze out at distances
greater than 1-3 AU from solar-type central stars; more precise estimates are
difficult to obtain due to uncertainties in the complex processes involved in
disk evolution, including dust growth, settling, and radial drift, and the
level of turbulence and viscous dissipation within disks. Interferometric
observations are now providing constraints on the positions of CO snow lines,
but extrapolation to the unresolved regions where water ice sublimates will
require much better theoretical understanding of mass and angular momentum
transport in disks as well as more refined comparison of observations with
sophisticated disk models.
