How does a thin wetted film dry up?

We study the thinning by evaporation of completely wetted water films on clean mica surfaces in a pure water vapor environment. In the thickness range from 100 to approximately 1000 {} the thinning is unstable to nucleation of dry patches, and the spreading of these patches invokes a ``dewetting'' of the substrate. The initial process is described by a simple nucleation theory. A number of hydrodynamic effects and instabilities are also observed at the boundary of the film.