The absorption of light by living organisms is important both as a probe of biochemical processes at the molecular level, and as the stimulus for myriad photobiological processes. Typically, light absorption may be characterized by measuring either the transmission or the reflectance spectrum; however, most biological systems in situ are not amenable to these measurements due to opacity, scattering, poorly defined or heterogeneous surface properties, etc. Thus, it is of interest to have a technique for measuring the absorption of light that is less constrained by the nature of the material under study. Photoacoustic spectroscopy (PAS) clearly meets this requirement while offering new information that arises uniquely from the combination of spectroscopic and calorimetric phenomena. In certain respects PAS is a qualitative spectroscopic technique, the spectra (except in special cases) are only similar to conventional absorption spectra; also for complex biological samples there is no general method of extracting extinction coefficients or concentrations from the observed signal. On the other hand, photophysical parameters such as quantum yields, lifetimes, and energies, characterizing the various excited states and relaxation pathways of photobiological systems in situ, can sometimes be measured by PAS.