Heats of mixing are the sensitive representation of the nonidealities of the liquid mixtures, which appear to be caused from the differences in molecular size, the type of molecular distribution and the interaction energy of molecules. The vapor-liquid equilibria as well as the heats of mixing for the liquid mixtures have been studied by many authors and the results of those studies were analyzed from the viewpoint of the nonidealities of the liquid mixtures. In this review it is intended to summarize mainly the theoretical background of the heats of mixing based on both the group solution theory and the association theory which were developed in recent years.