About one and a half decades ago, Feigenbaum, independently of Coullet and Tresser, observed that bifurcations, from simple dynamics to complicated ones, in a family of folding maps like quadratic polynomials follow an universal rule. This observation opened a new way to understanding transition from nonchaotic systems to chaotic or turbulent system in fluid dynamics and many other areas. The renormalization was used to explain this observed universality. This book is intended to bring the reader to the frontier of this active research area which is concerned with renormalization and rigidity in one dimensional dynamics. Most recent results and techniques developed by Sullivan and others (including the authors) in the past five years for an understanding of this universality as well as the most basic and important techniques in the study of one dimensional dynamics also included here.