The Endangered Species Act of 1973 is one of the nation's most significant and most controversial environmental laws. Over three-and-a-half decades, it has profoundly influenced both private and federal agency behavior. As the scope of that influence has come to be recognized, a law that is ostensibly to be guided by science has inevitably become entangled in politics. The generality of many of the law's key provisions has produced continuing uncertainty and conflict over some basic issues. Among these are what species or other taxa are potentially subject to the Act's protections, what the extent of those protections is, and whether the Act's ultimate goal of recovery is one that is being effectively achieved. New challenges face the administrators of this law, including that of incorporating climate change considerations into the decisions made under the Act, and responding to the information made available by recent advances in genetics. This paper provides a brief overview of the Endangered Species Act's history and its key provisions, and a more in-depth look at some of the current and recurrent controversies that have attended its implementation.