It is a well known fact that the properties and performance of polycrystalline materials, including polycrystalline thin films, are strongly affected by the grain structure. Therefore, in treating reactive phase formation in these films, it is (or it will inevitably be) necessary to quantify the grain structure of reactant and product phases and its evolution during the course of the reaction. Theoretical models and the conventional view of thin film reactions, however, have been largely extensions, to small and finite dimensions, of theories and descriptions developed for bulk diffusion couples. These models and descriptions primarily focus on the growth stage and to a much lesser extent on the nucleation stage. Consequently, these models and descriptions are not able to treat the development of product phase grain structure. Recent calorimetric investigations of several thin film systems demonstrate the importance of nucleation kinetics (and hence nucleation barriers) in product phase formation and provide quantitative measures of the thermodynamics and kinetics of formation of the product phases, thereby allowing some degree of comparison with reaction models. Furthermore, microstructural investigations of thin-film reactions demonstrate the non-planarity of the growth front and highlight the role of reactant-phase grain boundaries. In this paper, a summary of these experimental studies and recent theoretical treatments, which combine nucleation and growth in an integrated manner, is presented, with particular emphasis on the Nb/Al system. These experiments and models lead to a new view of reactive phase formation and grain structure evolution as one in which the latter is an integral part of the former. Based on this view, directions for future research are discussed.