Animal studies have been appealing to many area of psychology because, in comparison with human studies, they afford greater experimental control, more options for measuring physiological and genetic parameters, greater opportunities for naturalistic observation, and an accelerated life course. Thus, it makes sense that interest in and research on animal personality has flourished in recent years. Reviews of the literature show that: (a) personality exists and can be measured in animals; (b) studies of animal personality fall into three broad domains (animal-model research, behavioral ecology, and practical applications); (c) personality can be identified in a broad array of species; and (d) some traits show more cross-species generality than others. Conceptual and empirical analyses show that personality can be assessed in animals using rating and behavior-coding methods; comparisons of the two methods suggest that ratings are generally superior to behavior codings. Animal personality research is well placed to shed light on the genetic, biological, and environmental bases of personality and to illuminate research on personality development, personality perception, and the links between personality and health.