Population and community biology have been based largely on studies of abundant and widespread species. Most species are neither. This study draws together information from different disciplines on rarity. It draws general principles, focusing on the definition and measurement of rarity, its spatial and temporal dynamics, and its causes and interactions with conservation. Rare species are of particular concern to conservation because they tend to have a greater probability of extinction. This book should be of interest to conservation biologists, ecologists, environmental scientists and animal behaviourists.