We will review the role of classical variables crossing the Instability Strip in our understanding of galaxy formation and evolution. We will discuss the fundamental role of photometric surveys of resolved stellar populations in our Galaxy and beyond. We have recently analysed the old stellar populations, as traced by RR Lyrae stars, observed in low density environments such as the Galactic halo and dwarf galaxies surrounding the Milky Way. This investigation has revealed that small satellites can not have had a major role in building up this old component of the Galaxy, whereas the contribution of more massive dwarf galaxies like the Large Magellanic Cloud (LMC) and/or the still merging Sagittarius dwarf spheroidal galaxy can be significant. In the near future thanks to Gaia and to the Large Synoptic Survey Telescope these kind of studies will gain much more detail and will be largely extended to the the outskirts of our Local Group (distances 1Mpc).