Fundamental fact that any chemical reaction requires mobility of reactant molecules to allow their collisions, it seems that regularly observed amorphous phases play an active role at the first stages of solid-state reactions. These fluid or quasi-fluid intermediates act as a medium that enables mobility of involved chemical species, providing an environment in which a chemical reaction is possible. On the other hand, it is clear that solid-state chemical reactions are initiated by the contact of two substances and take place on the interface. However, this fundamental aspect has not been dealt with in sufficient detail so far. Within the framework of this project, penetration into the mechanistic details of these processes will be enabled by studying macroscopically static reactions on well-defined interfaces, and by their spatially and temporally resolved chemical mapping. On the other hand, time-resolved monitoring of selected macroscopically dynamic solid-state reactions, i.e. mechanochemical reactions, we will obtain results that will enable comparison of kinetic and other parameters that determine reactivity with those obtained by mapping static reactions. This will be made possible by extensive use of synchrotron X-ray powder diffraction and microspectroscopies. By this approach we will be able to qualitatively and quantitatively detect and characterize the evolution of chemical species involved in these fundamental processes. Based on these findings, we will discuss several important groups of reactions in the solid state, which are becoming more and more prevalent in modern preparative chemistry. In particular, we will investigate accelerated aging reactions, cocrystallizations from molecular eutectic mixtures and liquid-assisted grinding. Finally, we will apply all this to solid-state reactions relevant to technological, biomineralization and geological processes.