The research paper "Extinction risk from climate change" published in the journal Nature in January 2004 created front-page headlines around the world. The notion that climate change could drive more than a million species to extinction captured both the popular imagination and the attention of policy-makers, and provoked an unprecedented round of scientific critique.Saving a Million Species reconsiders the central question of that paper: How many species may perish as a result of climate change and associated threats? Leaders from a range of disciplines synthesize the literature, refine the original estimates, and elaborate the conservation and policy implications.