Extinction is a natural process. In geological time there have been several periods of mass extinction. One of these periods is unfolding right now but all the evidence suggests that current extinction rates are between a hundred and a thousand times greater than the background rate. To put this in to context, a quarter of all known mammalian species is at risk. The current extinction crisis is unique, because it is caused by the impact of one species, humans, on all others. This acceleration of species loss, and the much more widespread reductions in the populations of many species, is not merely a tragedy in aesthetics, it is also a threat to the quality of human life, indeed to the entire human enterprise. Biodiversity, the diversity of life, is not only fascinating and beautiful, it is the engine of all the world's natural cycles, and the source of many of the resources on which humanity depends. Concern about biodiversity conservation is, therefore, not merely the preoccupation of a few enthusiastic naturalists - it is the lifeline business of everybody. In this Very Short Introduction, David Macdonald introduces the concept of biodiversity and the basic biological processes that it involves - evolutionary, ecological, and behavioral. He considers the various threats to biodiversity, their impacts, and some of the solutions to the problems; concluding by considering the future of biodiversity conservation.