If we want to get the deep understandings for the behavior of thermodynamic quantities of solutions in connection with its microscopic structure, we must proceed to the collaboration of experimental study with theoretical ones. In this article, the outline of the integral equation theory of solutions is described, because the integral equation theory is the representative theory to describe the structure and thermodynamic properties of solutions on the same theoretical bases. At first, the basic properties of distribution functions are discussed. Next, the relations of thermodynamic quantities with the distribution functions and the integral equations on the distribution functions are summarized. Finally, calculated results are shown in the cases of excess enthalpies and partial molar enthalpies of binary mixtures as an example of an attempt to construct the intuitive image for the behavior of solutions on the rigorous theoretical basis.