This article reviews the role of solid–solid interface between phases and materials in photocatalytic reactions. We hypothesize that the solid–solid interface is a key structural feature that facilitates charge separation to hinder recombination and enhance photocatalytic efficiency, and may be the locus of defect sites that act as catalytic “hot spots”. The focus of this article is TiO 2-based photocatalytic materials and we present the work of others and some of our recent work synthesizing, characterizing, and testing TiO 2-based nanocomposites, especially mixed-phase titanium dioxide. The implications of this deeper understanding of structure–function relationships to energy applications are also discussed.