We review the theory and applications of the photoacoustic  and related methods belonging to the general area of photothermal  measurement techniques. This essay is intended for researchers especially unfamiliar with the basic ideas and potentialities of this rapidly developing field. The applications  of photothermal  techniques  include spectroscopic  applications,  such as infrared  and microwave  spectroscopies,  monitoring  of non-radiative  deexcitation processes, biological applications,  and measurements  of thermal properties.