Mammals range in body size from the gigantic blue whale to the tiny Etruscan shrew. Elephants and man may live for nearly one hundred years, while most shrews die before they are three months old. During the past decade, mammalogists and evolutionary biologists have begun to unravel the numerous factors that shape the enormous diversity of mammal life histories. In this volume, leading scientists provide a variety of perspectives on the newest theories in this active field of study. The principle uniting all studies of life history evolution is adaptation by natural selection. The first chapters in the book discuss this topic, offering evolutionary interpretations of geographic variation in mammal life histories, explaining how natural selection operates in fluctuating environments, introducing evolutionary predictions of demographic mathematics, and integrating life histories with behavioral ecology. The next chapters offer functional interpretations of the importance of body size in the life history. Next, several essays explain how developments in quantitative genetics have enabled us to distinguish between genetic and environmental components of variation within and between species. With this as a basis, the chapters that follow draw from principles of natural selection, allometry, and genetics to interpret differences among species of mammals. The book concludes with speculations on various areas where research seems most urgent for the development of a comprehensive understanding of mammal life history evolution. According to the authors, the field is rich with questions, and opportunities abound for both theoretical and empirical research.