This dissertation describes a hydrothermal method for synthesizing titanium dioxide nanocrystals with controllable physical properties and explores the influence of such properties on the material's photocatalytic efficiency. The preparation of nanoscale titania from an alkoxide precursor in ethanol under mild hydrothermal conditions yields highly crystalline, thermally stable, phase-pure anatase dots whose sizes can be fine-tuned through adjustment of reaction temperature, precursor concentration, and water-to-alkoxide ratio. By optimizing the synthetic conditions, one may obtain grain sizes as small as 5.5 nm and surface areas up to 250 m2 g-1. The importance of various physical properties in determining the photocatalytic performance of nanocrystalline titania is investigated and clarified using photodegradation of an azo dye as a model reaction. Experimental photocatalytic activities, as quantified by dye half-life and Langmuir-Hinshelwood rate constants, confirm that anatase is an inherently superior photocatalyst to rutile and that activity enhancement due to anatase-rutile synergy in mixed-phase catalysts is contingent upon other factors such as the nature of the anatase-rutile interface. Also, it is shown for the first time that the shape of a titania nanocrystal significantly affects its photocatalytic efficiency. Specifically, rodlike anatase nanocrystals with predominantly (1 0 1) surfaces perform poorly as photocatalysts because the non-dissociative adsorption of water to these surfaces prevents efficient generation of the OH• radicals thought to be essential for photocatalytic oxidation.