  On the basis of new theoretical results useful predictions concerning the
Period-Luminosity (PLR) and Period-Luminosity-Color (PLCR) relations both for
optical and infrared magnitudes are presented. It is shown that, following the
dependence of the instability strip on metallicity, there is a non negligible
dependence of the PLRs and PLCRs on the metallicity of the pulsating stars,
mainly for optical bands. In particular theoretical results predict a
dependence of the PLR on metals which is reversed with respect to current
empirical evaluations. To give a possible explanation for this discrepancy the
typical observational procedures used to estimate extragalactic distances
through Cepheid PLRs are here tested, with the aim of disentangling, if
possible, the reddening and metallicity effects. To this purpose, synthetic
PLRs for different metallicities were produced and treated as typical
observational samples.
