Calorimetry is one of the most powerful and convenient approaches for researching reactions accompanied by heat. Hence, researchers such as protein science, pharmaceutical science, and polymer science have been effectively applying calorimetric methods. We described the practical applications of calorimetric techniques to molecular interactions between proteins and small molecules such as drug candidates in terms of energetics. Given that no unified comprehensive summary of the structural thermodynamics important in understanding and designing molecular interactions and drug candidates is available, we first review structural thermodynamics based on calorimetric results in a concise way. The basis of advanced applications of empirical relations to binding reactions is described following a general and simple means of using isothermal titration calorimetry (ITC) data. We further describe our recent applications of ITC results and structural thermodynamics to protein-protein interactions. Second, the general energetic nature of molecular binding events is explained by using databases containing a number of case studies obtained by ITC and statistical analysis. Based on these trends and experimental efforts manipulating entropy and enthalpy, thermodynamic optimization for selecting and designing drug candidates has been improved. Well-rounded exploitation of calorimetry with structural thermodynamics is key to increasing the current understanding of molecular interactions.