The article provides information about photoacoustic spectroscopy, a form of spectroscopy that uses light and sound to probe the behavior of materials. This form is part of a class of photothermal techniques, in which an impinging light beam is absorbed and changes the thermal state of the object under observation. One advantage to this form of spectroscopy is that it can be performed on all phases of matter. Another is that this form does not have to be dissolved in some solvent or embedded in a solid-state matrix.