RR Lyrae stars as probes of the Milky Way structure and formation
  RR Lyrae stars being distance indicators and tracers of old population serve
as excellent probes of the structure, formation, and evolution of our Galaxy.
Thousands of them are being discovered in ongoing wide-field surveys. The OGLE
project conducts the Galaxy Variability Survey with the aim to detect and
analyze variable stars, in particular of RRab type, toward the Galactic bulge
and disk, covering a total area of 3000 deg^2. Observations in these directions
also allow detecting background halo variables and unique studies of their
properties and distribution at distances from the Galactic Center to even 40
kpc. In this contribution, we present the first results on the spatial
distribution of the observed RRab stars, their metallicity distribution, the
presence of multiple populations, and relations with the old bulge. We also
show the most recent results from the analysis of RR Lyrae stars of the Sgr
dwarf spheroidal galaxy, including its center, the globular cluster M54.
