The lecture discusses the unique value of solution calorimetry for the resolution of a variety of problems in physical organic chemistry. The past fifty years have been a Golden Age of enlightenment in providing a clear picture of how and why organic chemical reactions take place. Through the application of simplified notions of the transition state, and a carefully developed characterization of high energy intermediates (e.g. radicals, carbenium ions, and carbanions), organic chemistry developed a language of electron transfer mechanisms so powerful that college sophomores taking their introductory organic course became able to solve problems and make predictions that would have baffled Nobel Laureates in the 1920s. Although most of the fundamental structure/energy relationships through which organic chemistry has been reduced to a rational science have depended on free energy measurements (rates and equilibria), many fundamental problems could be attacked more directly by determination of heat changes using calorimetry. The failure of most organic chemists and most calorimetrists to exploit these opportunities has been somewhat puzzling, but has provided the lecturer and his students with a relatively unchallenged niche from which to attack some fundamental organic chemical problems that will be reviewed in the presentation.