There are a great many substances, both organic and inorganic, that, because of their physical state, cannot be readily studied by conventional absorption or reflection techniques. In photoacoustic spectroscopy, light absorbed by the sample is converted into a measurable acoustic signal, and spectra closely corresponding to optical absorption spectra, can be obtained on any type of solid or semisolid material, whether it be crystalline, powder, smear, gel, etc. The physical processes underlying the photoacoustic effect in solids, and a mathematical analysis of the production of the photoacoustic signal will be presented. The methodology of photoacoustic spectroscopy will also be described. Various applications of this technique in the fields of physics, chemistry, biology, and medicine will be discussed. These applications will be treated under three main headings: bulk studies, surface studies, and de-excitation studies.