Non perturbative phenomena play often a prominent role in equilibrium and
out of equilibrium phase transitions. The reason is twofold: either these
transitions correspond to the strong coupling regime of the field theories
describing them or genuinely non perturbative excitations (topological
defects, bound states, etc) drive the transition. In both cases, using
perturbation theory becomes problematic. We describe on the example of
reaction-diffusion processes how the non perturbative renormalization group
formalism can be implemented for out of equilibrium systems. We show how to
determine both universal and non universal properties (phase diagrams) of some
branching and annihilating random walks in all dimensions.