Semiconductor photoelectrochemistry experienced a renaissance stimulated by the development of mesoporous dye-sensitized solar cells following the 1991 Nature paper of O'Regan and Grätzel. The resulting move away from well-defined single crystal bulk materials to high surface area nanostructured electrodes opened a new field of research, and many of the ideas that had been developed for bulk semiconductor electrodes required re-examination in view of the very different length scales. Nanostructured semiconductor electrodes are now also being utilized for light-driven water splitting and environmental remediation. This historical development has resulted in a convergence of the fields of semiconductor photoelectrochemistry and photocatalysis at semiconductor particles. The objective of this chapter is to review the basic ideas that were developed originally to understand the photoelectrochemical behaviour of bulk semiconductors and to see how these ideas need to be modified when considering nanostructured semiconductor electrodes and dispersed colloidal systems.