Inbreeding, the mating of close kin, and outbreeding, the mating of distant relatives or unrelated organisms, have long been important subjects to evolutionary biologists. Inbreeding reduces genetic diversity in a population, increasing the likelihood that genetic defects will become widespread and deprive a population of the diversity it may need to cope with its environment. Most plants and animals have evolved behavioral and morphological mechanisms to avoid inbreeding. However, today many endangered species exist only in small, very isolated populations where inbreeding is unavoidable, so it has become a concern for conservationists. In this volume, twenty-six experts in evolution, behavior, and genetics examine the causes and consequences of inbreeding. The authors ask whether inbreeding is as problematic as biologists have thought, under what ecological conditions inbreeding occurs, and whether organisms that inbreed have mechanisms to dampen the anticipated problems of reduced genetic variation. The studies, including theoretical and empirical work on wild and captive populations, demonstrate that many plants and animals inbreed to a greater extent than biologists have thought, with variable effects on individual fitness. Graduate students and researchers in evolutionary biology, animal behavior, ecology, and conservation biology will welcome this wide-ranging collection.