The crystallization of anatase from amorphous titania has been controlled using the hydrothermal technique. Crystallite size and surface area can be controlled by careful alterations of the hydrothermal conditions. The catalytic effect of the water on crystallization has been demonstrated, and the crystallization mechanism has been shown to change from a solid-state type to dissolution precipitation as the temperature is increased. The influence of the amorphous precursor preparation route has been examined in order to evaluate the effect of contaminant ions on the crystallization process. It has been shown that the chloride ion accelerates the nucleation of the anatase, even under dry conditions. The effects of acidic and basic solutions have been studied. Acidic conditions result in the formation of anatase, brookite, and rutile, whereas basic conditions accelerate the production of anatase. The mechanisms for the hydrothermal crystallization of anatase and rutile are discussed.