This is a comprehensive account of RR Lyrae stars, and traces the story from their initial discovery a century ago, through to their present status. This book reviews our current understanding of RR Lyrae stars. It is a unique explanation of the multiple applications of these variable stars for a range of astrophysical problems. Horace A. Smith describes the use of RR Lyrae stars as probes of old stellar populations, both in the Milky Way and other galaxies, and as an outstanding testing ground for stellar evolution and pulsation theories. He stresses the significance of variable stars for our ultimate understanding of the history and scale of the Milky Way and nearer extragalactic systems. For advanced students and researchers of astronomy, this is a definitive account of the modern theories surrounding RR Lyrae stars.