  We develop a variational mean-field theory of the ferromagnetic transition in
compounds like Lanthanum-Manganite within the framework of the Double-Exchange
Model supplemented by modest disorder. We obtain analytical expressions for the
transition temperature, its variation with the valence electron-density and its
decrease with disorder.
  We derive an expression for the conductivity for both the paramagnetic and
the ferromangetic metallic phases, and study its dependence on the temperature
and magnetic field. A simple relation between the resistivity in the
ferromagnetic phase and the spontaneous magnetization is found. Our results are
in a good agreement with the experimental data on transition temperatures and
resistivity in the manganite compounds with relatively small disorder. We
comment on the effects of increased disorder.
