  We discuss the relationships between large deviations in stochastic systems,
and "effective interactions" that induce particular rare events. We focus on
the nature of these effective interactions in physical systems with many
interacting degrees of freedom, which we illustrate by reviewing several recent
studies. We describe the connections between effective interactions, large
deviations at "level 2.5", and the theory of optimal control. Finally, we
discuss possible physical applications of variational results associated with
those theories.
