  We review works on screening of a macroion, such as colloidal particle or
double helix DNA, by multivalent counterions. Multivalent metal ions, charged
micelles, short or long polyelectrolytes can play the role of multivalent
counterions. Due to the strong Coulomb repulsion such multivalent counterions
form a strongly correlated liquid resembling a Wigner crystal at the surface of
the macroion. Even if the macroion is neutralized by this liquid, a newly
arriving counterion creates in the liquid a correlation hole or image which
attracts the ion to the surface. As a result, total charge of adsorbed
counterions can substantially exceed the bare macroion charge, so that the
macroion net charge inverts its sign. We discuss two previously suggested
explanations of charge inversion and show that physics underlying both of them
is intrinsically that of correlations, so that correlation is the universal
force driving charge inversion.
