The twentieth anniversary of Earth Day in 1990 indicated that environmentalism has become an enduring and successful social movement and thus has avoided the rapid demise that many movements experience. However, the U.S. environmental movement has changed considerably since 1970, most notably by becoming a much more diverse movement. The contributions to this volume describe the major strands of environmentalism that have developed over the past two decades and, in the process, provide an overview of the evolution of the movement.