Photoacoustic calorimetry (PAC) has emerged as an important tool for obtaining thermodynamic information, including enthalpy and molar volume changes, associated with photo-initiated processes for both chemical and biological systems on the nanosecond to microsecond time scale. This technique can provide energetic information, specifically enthalpies, critical to fully understanding reaction mechanisms in chemical and biological reaction mechanisms provided the process can be photo-initiated. In addition to enthalpies, PAC data gives molar volume changes accompanying the enthalpy changes. Changes in molar volume can provide key insights into the relationship between solvent and reaction intermediates. Here, the theory behind PAC is presented as well as several examples of applications of PAC to biological and chemical systems.