  We present a detailed comparison between predicted and empirical PL_{I,K}
relations and Wesenheit function for Galactic and Magellanic Clouds (MCs) First
Overtone (FO) Cepheids. We find that zero-points predicted by Galactic Cepheid
models based on a noncanonical (mild overshooting) Mass-Luminosity (ML)
relation are in very good agreement with empirical zero-points based on
HIPPARCOS parallaxes, while those based on canonical (no overshooting) ML
relation are about 0.2-0.3 mag brighter. We also find that predicted and
empirical PL_K relation and Wesenheit function give, according to optical (V,I
OGLE) and near-infrared (NIR, K, 2mass) data, mean distances to the MCs that
agree at the 2% level. Individual distances to the Large and the Small Cloud
are: 18.53+-0.08-19.04+-0.11 (theory) and 18.48+-0.13-19.01+-0.13 (empirical).
Moreover, predicted and empirical FO relations do not present, within the
errors, a metallicity dependence. Finaly, we find that the upper limit in the
FO period distribution is a robust observable to constrain the accuracy of
pulsation models. Current models agree within 0.1 in log P with the observed FO
upper limits.
