  We investigate non-equilibrium critical phenomena using a nonperturbative
renormalization group method. Reaction-diffusion processes are described by a
scale dependent effective action which evolution is governed by very generic
flow equations, that are derived. They allow to recover the critical exponents
of directed percolation, and moreover to calculate the microscopic reaction
rates which give rise to a phase transition in the case of branching and
annihilating random walks with odd number of offsprings, even in three
dimensions.
