  These lectures provide an introduction to the theory of disordered
interacting electron systems. In particular, we concentrate on those aspects
which are fundamental for the problem of the metal-insulator transition due to
the interplay of disorder and interaction. After reviewing the problem of
disordered non-interacting electrons, we examine the past and recent
experimental urgency to take into account interaction effects. We describe, by
using the language of standard perturbation theory, how these interactions
effects lead to the picture of the renormalized disordered Fermi liquid. This
allows us to obtain the renormalization group equations for the disordered
interacting electron liquid. The group equations are then discussed by the
light of the existing experimental evidence.
