How many species face extinction due to climate change? This is one of the most pressing questions of climate change biology. It is featured in front-page stories and on the evening news. “More than a million species wiped out by climate change” and “One third of all of the world's plants and animals gone due to climate change” are examples of some of the headlines. What is the science behind these sensational claims, and how do we know whether climate change poses a major extinction risk? This chapter provides answers to these questions.