  Classical Cepheid and RR Lyrae variables are radially pulsating stars that
trace young and old-age stellar populations, respectively. These classical
pulsating stars are the most sensitive probes for the precision stellar
astrophysics and the extragalactic distance measurements. Despite their
extensive use as standard candles thanks to their well-defined
Period-Luminosity relations, distance measurements based on these objects
suffer from their absolute primary calibrations, metallicity effects, and other
systematic uncertainties. Here, I present a review of classical Cepheid, RR
Lyrae, and Type II Cepheid variables starting with a historical introduction
and describing their basic evolutionary and pulsational properties. I will
focus on recent theoretical and observational efforts to establish absolute
scale for these standard candles at multiple wavelengths. The application of
these classical pulsating stars to high-precision cosmic distance scale will be
discussed along with observational systematics. I will summarize with an
outlook for further improvements in our understanding of these classical
pulsators in the upcoming era of extremely large telescopes.
