Cepheid and RR Lyrae Variables as Standard Candles and What Else?

Classical Cepheid and RR Lyrae variables are fundamental tracers of cosmic distances and stellar evolution and pulsation. Light curve analysis and pulsation properties of these radially pulsating stars provide stringent tests for theoretical evolution and pulsation models. I discuss a detailed light curve analysis of classical Cepheid variables in the Galaxy and the Magellanic Clouds at multiple wavelengths. The variation of light curve parameters as a function of period, wavelength and metallicity is quantified to constrain the theoretical parameters, such as, the mass-luminosity relations obeyed by Cepheids. I briefly summarize the application of multiwavelength light curve data for Type II Cepheid and RR Lyrae variables to study the Milky Way bulge.