  The Cepheids' P-L relation got itself talked about again in the recent past
due to claims of observed metallicity dependences of at least its zero point.
The divergences of the observational results obtained by the different authors
are large, however. Therefore, to date the situation concerning the magnitude
and the reliability of the effect is rather uncertain on the observational
side. Along the theoretical avenue, some progress towards more realistic and in
particular more consistent Cepheid modeling has been achieved recently. Upper
limits on the reaction of the P-L relation on assumptions in evolution
computations, pulsation stability analyses, and mappings onto photometric
passbands can be estimated now quite reliably. This informal review discusses
some of the progress (relying mostly on an extensive comparison by Sandage et
al. 1998) and it emphasizes modeling problems which still have to be overcome
to finalize the theoretical foundation of the Cepheids' P-L relation.
