  We develop a Renormalization Group (RG) approach to the study of existence
and uniqueness of solutions to stochastic partial differential equations driven
by space-time white noise. As an example we prove well-posedness and
independence of regularization for the $\phi^4$ model in three dimensions
recently studied by Hairer. Our method is "Wilsonian": the RG allows to
construct effective equations on successive space time scales. Renormalization
is needed to control the parameters in these equations. In particular no theory
of multiplication of distributions enters our approach.
