The thermodynamic approach to the understanding of martensitic transformations can provide insight into the mechanism of transformation, serve as a generalization technique among the alloying systems, and as a framework for the synthesis of data on kinetic, structural, crystallographic, and other aspects of the transformation. The quality and utility of existing thermochemical models and data are examined. The approach requires long, unverifiable extrapolation of the thermodynamic functions and must be used with circumspection. On the other hand, much that is known is derived from a thermodynamic viewpoint. The present authors are not hopeful of great breakthroughs in the quality of the thermodynamic data. More significant, it appears, would be attempts to formally cast the areas of crystallographic analysis (transformation strain energy) and the like into the thermodynamic framework.