  We explore the possibility of achieving one--step unification of the standard
model coupling constants within non supersymmetric and supersymmetric gauge
models, which at low energies have only the standard particle content. The
constraints are the experimental values of $\alpha_{em}$, $\alpha_s$ and
$\sin^2\theta_W$ at $10^2 GeV$, and the lower bounds for FCNC and proton decay
rates. The analysis is done in a model independent way.
