  The theoretical period-luminosity relation for Cepheids was deduced from
stellar evolution and pulsation computations. Questions concerning the
metallicity dependence of the relation as well as the constancy of the
pulsation `constant' Q in the instablility strip were addressed. We found no
significant metallicity dependence of either the slope or of the zero-point of
the period-luminosity relation. The Q-value, however, varies across the
instability strip, in effective temperature as well as in luminosity; its
metallicity dependence, on the other hand, is marginal.
