  We provide a microscopic description of the magnetic properties of UGe$_2$
and in particular, of its both classical and quantum critical behavior. Namely,
we account for all the critical points: the critical ending point (CEP) at the
metamagnetic phase transition, the tricritical point, and the quantum critical
end point at the ferromagnetic to paramagnetic phase transition. Their position
agrees quantitatively with experiment. Additionally, we predict that the
metamagnetic CEP can be traced down to zero temperature and becomes quantum
critical point by a small decrease of both the total electron concentration and
the external pressure. The system properties are then determined by the quantum
critical fluctuations appearing near the instability point of the Fermi surface
topology.
