  With ages comparable to the age of the Universe, the variable stars of RR
Lyrae type have eyewitnessed the formation of their host galaxies, and thus can
provide information on the processes that led to the assembling of large
galaxies such as the Milky Way and the Andromeda galaxy. The present knowledge
of the RR Lyrae population in Local Group dwarf spheroidal galaxies is
reviewed,calling attention to the "ultra-faint" spheroidal systems recently
discovered around the Milky Way by the Sloan Digital Sky Survey. The properties
of the RR Lyrae stars and the Oosterhoff dichotomy observed for Galactic
globular clusters and field RR Lyrae stars are discussed, since they put
constrain on the possibility that the Milky Way and Andromeda halos were built
up from protogalactic fragments resembling the dwarf spheroidals we observe
today.
