  We present a model for the ion distribution near a charged surface, based on
the response of the ions to the presence of a single test particle. Near an
infinite planar surface this model produces the exact density profile in the
limits of weak and strong coupling, which correspond to zero and infinite
values of the dimensionless coupling parameter. At intermediate values of the
coupling parameter our approach leads to approximate density profiles that
agree qualitatively with Monte-Carlo simulation. For large values of the
coupling parameter our model predicts a crossover from exponential to algebraic
decay at large distance from the charged plate. Based on the test charge
approach we argue that the exact density profile is described, in this regime,
by a modified mean field equation, which takes into account the interaction of
an ion with the ions close to the charged plate.
