The PAC webpages are arranged in three levels of increasing complexity. The first level, where you are now, is an introduction to the technique and applications of photoacoustic calorimetry. Equations and schemes are kept to a minimum and presented in a simplified manner. The second level, aimed at the more curious readers, students, and specialists, is accessed in the More Details chapter. That level contains a more thorough and accurate description of PAC. The third level, intended for those who use photoacoustic calorimetry for investigating free radical reactions in solution, is found in For PAC Users chapter. There we present all the equations and auxiliary data needed, and our own results on the subject.