Classical Cepheids and RR Lyrae stars similar, but not too much.

We discuss evolutionary and pulsation properties of classical Cepheids and RR Lyrae stars. We focus our attention on the role that they play as standard candles and as tracers of young and old stellar populations. We also introduce some plain physical argument concerning the occurrence of optical/near-infrared Period-Luminosity relations in classical Cepheids and in RR Lyrae. Moreover, we outline pros and cons of the different diagnostics adopted to estimate individual distances, and in particular, the uncertainties affecting reddening and chemical compositions. We also address some relevant issues concerning optical and Near-Infrared (NIR) spectroscopy of radial variables. Finally, we briefly outline near future experiments concerning Galactic and extra-galactic stellar variability projects.