  We propose a simple mechanism for solving the $\mu$ problem in the context of
minimal low--energy supergravity models. This is based on the appearance of
non--renormalizable couplings in the superpotential. In particular, if $H_1H_2$
is an allowed operator by all the symmetries of the theory, it is natural to
promote the usual renormalizable superpotential $W_o$ to $W_o+\lambda W_o
H_1H_2$, yielding an effective $\mu$ parameter whose size is directly related
to the gravitino mass once supersymmetry is broken (this result is maintained
if $H_1H_2$ couples with different strengths to the various terms present in
$W_o$). On the other hand, the $\mu$ term must be absent from $W_o$, otherwise
the natural scale for $\mu$ would be $M_P$. Remarkably enough, this is entirely
justified in the supergravity theories coming from superstrings, where mass
terms for light fields are forbidden in the superpotential. We also analyse the
$SU(2)\times U(1)$ breaking, finding that it takes place satisfactorily.
Finally, we give a realistic example in which supersymmetry is broken by
gaugino condensation, where the mechanism proposed for solving the $\mu$
problem can be gracefully implemented.
