  We investigate the quantum phase transition of itinerant ferromagnets. It is
shown that correlation effects in the underlying itinerant electron system lead
to singularities in the order parameter field theory that result in an
effective long-range interaction between the spin fluctuations. This
interaction turns out to be generically {\em antiferromagnetic} for clean
systems. In disordered systems analogous correlation effects lead to even
stronger singularities. The resulting long-range interaction is, however,
generically ferromagnetic.
  We discuss two possibilities for the ferromagnetic quantum phase transition.
In clean systems, the transition is generically of first order, as is
experimentally observed in MnSi. However, under certain conditions the
transition may be continuous with non-mean field critical behavior. In
disordered systems, one finds a very rich phase diagram showing first order and
continuous phase transitions and several multicritical points.
