‘Why extinctions happen’ looks at what we currently know about causes and styles of extinction today and in the past. This knowledge lies at the heart of our understanding about the way the Earth’s environment works and what happens at times of extremes. The importance of the species–area relationship is discussed and it is shown that species with large and widely dispersed populations are at risk of extinction as well as those with small and narrowly dispersed populations. Extinction today is being demonstrably caused by human activities, such as habitat destruction, the introduction of invasive species, and over-exploitation of natural resources. Future climate change is an additional potential cause of extinction.