  In many domains of physics, methods are needed to deal with non-perturbative
aspects. I want here to argue that a good approach is to work on the Borel
transforms of the quantities of interest, the singularities of which give
non-perturbative contributions. These singularities in many cases can be
largely determined by using the alien calculus developed by Jean \'Ecalle. My
main example will be the two point function of a massless theory given as a
solution of a renormalization group equation.
