RR Lyrae stars: prime calibrators of the first rung of the distance ladder

Abstract RR Lyrae variables are the primary standard candles for old stellar populations, and the traditional first step in the definition of the distance scale. Their properties are known on the basis of well-established physical concepts and their calibration is based on several empirical methods. Both aspects are critically reviewed, and their application as distance indicators within the Galaxy and the Local Group are discussed, also in view of the observing facilities that will be available in the near future.