Reactive events such as conformation change of macromolecules, chemical reactions in solution, nucleation events during phase transitions, thermally induced magnetization reversal in micromagnets, etc. pose challenges both for computations and modeling. At the simplest level, these events can be characterized as the hopping over a free energy barrier associated with the motion of the system along some reaction coordinate. Indeed this is the picture underlying classical tools such as transition state theory or Kramers reaction rate theory, whose results fit the mathematical framework of the theory of large deviations. These approaches have been successful to explain reactive events in a wide variety of context. However they presuppose that we know or can guess beforehand what the reaction coordinate of the event is. In many systems of interest -- protein folding, enzyme kinetics, protein-protein interactions, etc. -- making such educated guesses is hard if not impossible. The question then arises whether we can develop a more general framework to describe reactive events, elucidate their pathway and mechanism, and give a precise meaning to a concept such as the reaction coordinate. In this talk I will discuss recent theoretical advances that have been made in this context, and how they can be used to design efficient algorithms to compute the pathway and rate of reactive events.