  We present a new theoretical approach for the study of the phase diagram of
interacting quantum particles: bosons, fermions or spins. In the neighborhood
of a phase transition, the expected renormalization group structure is
recovered both near the upper and lower critical dimension. Information on the
microscopic hamiltonian is also retained and no mapping to effective field
theories is needed. A simple approximation to our formally exact equations is
studied for the spin-$S$ Heisenberg model in three dimensions where explicit
results for critical exponents, critical temperature and coexistence curve are
obtained.
