When Congress passed the Endangered Species Act in 1973, it did so in the belief that threats to emblematic creatures like bison or commercial crops such as fish could be resolved simply by placing limits on hunting and trade. Few lawmakers could have foreseen how the implementation of that law would change over the years, as the directive to save seemingly insignificant species would extend to the protection of entire ecosystems. As the courts have interpreted the law in the light of a better understanding of science, the ESA increasingly has become a tool used to block development projects and restrict private land use. Shannon Petersen has written a political and legal history of the Endangered Species Act that explains how and why this piece of legislation has become so controversial. This book demonstrates that litigation and judicial decision-making have played a far more important role in shaping the modern environmental movement than has been previously thought. It focuses on two major controversies that developed over the implementation of the Act: a Supreme Court-ordered delay in dam construction when a fish called the snail darter was declared endangered and a threat to logging in the Northwest in the interest of saving the spotted owl. In reviewing these seminal cases, Petersen demonstrates the complex interplay among government, science, environmentalism, and natural resource industries in shaping endangered species policy in America. The ESA is now over a quarter-century old and remains the broadest and most powerful law in the world for the protection of species; yet, as Petersen observes, it remains unclear whether the law has been successful. Acting for endangered species is a story of strategic lawsuits, broad judicial interpretations, and administrative retrenchment that have left the ESA itself on the brink of extinction, and it demonstrates the extent of environmental policy dilemmas in America today.