This book forges a synthesis between the discipline of behavioural ecology and what is known of the social behaviour and population dynamics of cyclic populations of vertebrates, with particular discussion of the cyclic voles and lemmings. Areas of uncertainty, and those which show promise are identified, and tests of competing hypotheses are suggested. Finally, the relevance of social dynamics to population growth and decline are examined, and a hypothesis on the role of aggressiveness in expanding populations is presented.