Difficulties are briefly described in measurements of interface phenomena. The principle of photoacoustic spectroscopy (PAS) is introduced by explaining energy transfer routes. Light absorbed at an interface produces light (fluorescence), chemical species, and heat. PAS utilizes the heat generated after light absorption by detecting the pressure change due to the temperature rise. Therefore, PAS is very insensitive to the elastic scattering of light, which causes serious difficulties in conventional spectroscopic methods applied to inhomogeneous systems, i.e., interfaces. Instrumental features are explained especially for a detection cell. For a gas-solid interface system, a sensitive microphone placed on the wall of a sample chamber is used to detect the heat emitted as sound at an interface. For a liquid-solid interface system, a piezoelectric ceramic is utilzed as a pressure sensor and also as part of the cell for the liquid. Recent results are described for in situ observation of spectral sensitizing dyes at a semiconductor electrode-solution interface and at a semiconductor-gas interface.