Soay Sheep synthesises one of the most detailed studies of demography and dynamics in a naturally regulated population of mammals. Unlike most other large mammals, the Soay sheep population of Hirta in the St Kilda archipelago show persistent oscillations, sometimes increasing or declining by more than 60 per cent in a year. Soay Sheep explores the causes of these oscillations and their consequences for selection on genetic and phenotypic variation within the population, drawing on studies over nearly twenty years of the life-histories and reproductive careers of many individuals. Covering population dynamics, demography and their effects on selection, energetic and resource limitations on the interaction between sheep and parasites, and the adaptive significance of their reproductive characteristics, it provides unique insights into the regulation of other herbivore populations and the effects of environmental change on selection and adaptation. It will be essential reading for vertebrate ecologists, demographers, evolutionary biologists and behavioural ecologists.