  We have computed stellar evolutionary models for stars in a mass range
characteristic of Cepheid variables ($3<m/\Msol<12$) for different
metallicities representative of the Galaxy and the Magellanic Clouds
populations. The stellar evolution calculations are coupled to a linear non
adiabatic stability analysis to get self-consistent mass-period-luminosity
relations. The period - luminosity relation as a function of metallicity is
analysed and compared to the recent EROS observations in the Magellanic Clouds.
The models reproduce the observed width of the instability strips for the SMC
and LMC. We determine a statistical P-L relationship, taking into account the
evolutionary timescales and a mass distribution given by a Salpeter mass
function. Excellent agreement is found with the SMC PL relationship determined
by Sasselov et al. (1997). The models reproduce the change of slope in the P-L
relationship near $P\sim 2.5$ days discovered recently by the EROS
collaboration (Bauer 1997; Bauer et al. 1998) and thus explain this feature in
term of stellar evolution. Some discrepancy, however, remains for the LMC
Cepheids. The models are also in good agreement with Beat Cepheids observed by
the MACHO and EROS collaborations. We show that most of the 1H/2H Beat Cepheids
have not yet ignited central helium burning; they are just evolving off the
Main Sequence toward the red giant branch.
