Variable stars as standard candles and stellar tracers
  We review the current status of classical Cepheid and RR Lyrae distance scales. We discuss the most recent theoretical and empirical findings together with pros and cons of different methods. The use of Cepheids and RR Lyrae stars as stellar tracers are also discussed. Preliminary findings concerning the impact that GAIA will have on the calibration of distance determinations and astrophysical parameters are also presented. 
G. Bono (1), M. Cignoni (2) ((1) INAF - Osservatorio Astronomico di   Roma, (2) Dipartimento di Fisica "E. Fermi" Pisa)