The US environmental movement has proven to be exceptionally successful and enduring, as demonstrated by the 20th anniversary of Earth Day in 1990. The success of Earth Day 1990 indicates that the environmental movement is not only alive and well, but after two decades it may be stronger than ever. Few social movements achieve such widespread acceptance and fewer still are able to celebrate a 20th anniversary. Why has environmentalism been able to avoid the fate of most short-lived movements and how has it changed since the first Earth Day? Such questions are the focus of this book, which is intended to provide an overview of the evolution of US environmentalism during the past two decades.