Infrared spectroscopy has been developed as a powerful tool for the in situ study of the electrochemical interface. The possibility of using a multiple-internal-reflection geometry makes it especially suitable for the study of the semiconductor \ electrolyte interface. For high sensitivity to surface species, it is implemented as a differential or a modulation technique. Absorption due to vibrational transitions provides straightforward characterisation of the changes in the chemical state of the surface (interfacial films, adsorbed species, structure of the double layer); it can also be used to probe the composition of the electrolyte in the diffusion layer (redox species, reaction intermediates, pH) or even of the bulk of the electrode, in the case of intercalation reactions. Electronic absorption provides direct information on the semiconductor space-charge layer and on electronic states at the interface. Inherent possibilities of the infrared technique include the use of the polarisation of the infrared light, which gives information on the orientation of interface species, and the analysis of the time response of the infrared absorption, which gives direct information on the interfacial kinetics. Careful analysis of the changes in electrolyte absorption and spectrum baseline may provide valuable indirect information, for instance about infrared non-active species, surface roughening, or the formation of a porous layer at the interface. In situ infrared spectroscopy clearly offers a wealth of information about the semiconductor I electrolyte interface, and it is sufficiently convenient and versatile that it could be used to a much wider extent than has been the case so far.