How did rodent outbreaks in Germany help to end World War I? What caused the destructive outbreak of rodents in Oregon and California in the late 1950s, the large population outbreak of lemmings in Scandinavia in 2010, and the great abundance of field mice in Scotland in the spring of 2011? Population fluctuations, or outbreaks, of rodents constitute one of the classic problems of animal ecology, and this book sifts through the last eighty years of research to draw out exactly what we know about rodent outbreaks and what should be the agenda for future research. The author has synthesized the research in this area, focusing mainly on the voles and lemmings of the Northern Hemisphere—his primary area of expertise—but also referring to the literature on rats and mice. He covers the patterns of changes in reproduction and mortality, the mechanisms that cause these changes—including predation, disease, food shortage, and social behavior—and how landscapes can affect population changes, methodically presenting the hypotheses related to each topic before determining whether or not the data supports them. The author ends on an expansive note, by turning his gaze outward and discussing how the research on rodent populations can apply to other terrestrial mammals.