  The present work is devoted to the derivation of an effective
magnon-paramagnon theory starting from a microscopic lattice model of
ferromagnetic metals. For some values of the microscopic parameters it
reproduces the Heisenberg theory of localized spins. For small magnetization
the effective model describes the physics of weak ferromagnets in accordance
with the experimental results. It is written in a way which keeps O(3) symmetry
manifest,and describes both the order and disordered phases of the system.
  Analytical expression for the Curie temperature,which takes the magnon
fluctuations into account exactly, is obtained. For weak ferromagnets $T_c$ is
well below the Stoner's critical temperature and the critical temperature
obtained within Moriya's theory.
