This introduction to the renormalization group, an edited and corrected second edition, discusses examples from diverse areas of physics. Designed for a one-semester course for advanced graduate students, the treatment requires a solid background in classical mechanics, statistical mechanics, and quantum mechanics. The text begins with an examination of self-similarity and scale invariance, followed by chapters on the renormalization group approaches to chaos and percolation, renormalization group and critical phenomena, and an extensive treatment of the Ising model. Additional topics include mean field theory and the Gaussian fixed point, the spherical model and the 1/n expansion, the two-dimensional X-Y model and the Kosterlitz-Thouless transition, and other subjects. Each chapter is augmented by problems and references, and three helpful Appendixes supplement the text.