During the past 25 years, independent regulatory agencies have become widespread institutions for regulatory governance. This book studies how they have diffused across Europe and compares their formal independence in 17 countries and seven sectors. Through a series of quantitative analyses, it finds that governments tend to be more prone to delegate powers to independent regulators when they need to increase the credibility of their regulatory commitments and when they attempt to tie the hands of their successors. The institutional context also matters: political institutions that make policy change more difficult are functional equivalents of delegation. In addition to these factors, emulation has driven the diffusion of independent regulators, which have become socially valued institutions that help policymakers legitimize their actions, and may even have become taken for granted as the appropriate way to organize regulatory policies.