This chapter aims to explain sintering and redispersion in terms of the thermodynamics of wetting and to identify conditions under which sintering by migration of crystallites or sintering by Ostwald ripening occurs. It shows that the Ostwald ripening mechanism is associated with a particular type of phase separation. Supported metal crystallites are composed of a large number of small metal crystallites having sizes between 2 and a few tens of nm distributed over a substrate. The supported metal crystallites are of interest in crystal growth, in thin solid film technology and in catalysis. Starting from metal crystallites, the heating in oxygen generates metal oxide which spreads through the leading edge of the crystallites over the substrate. As long as the surface concentration in the film which is formed is lower than the saturation concentration corresponding to the largest crystallite present, all the crystallites will lose molecules to the substrate.