The reaction force F(R) of a chemical or physical process is the negative derivative of the system's potential energy V(R) along the reaction coordinate. The features of F(R) – its maxima, minima and zeroes – divide the process into well-defined stages which can, in general, be characterized in terms of changes in structural and/or electronic properties. This has been demonstrated for bond dissociation/formation and for reactions that have activation barriers in both forward and reverse directions. An important aspect of the reaction force is that it naturally and unambiguously divides activation energies into two components, one corresponding to the preparative structural stage of the process and the other to the first phase of the transition to products. It is shown how this can help to elucidate the effect of a solvent or a catalyst upon an activation barrier.