Individual animals show consistent differences in their behavioral tendencies. Some individuals are generally bolder, shyer, or more aggressive than others. This phenomenon is termed as animal personality or behavioral syndrome, and it has been observed in a wide range of animal species. In this article, we review the personality concepts and methodologies that has been used in two major study fields on this topic, i.e. the animal personality psychology and the behavioral syndrome research. Then, we shed light on how we can understand consistency in individual behavioral tendencies and how we can describe individual differences in animal behavior, by focusing on the differences and similarities between two study fields. Finally, we emphasize the importance of evolutionary framework for an integrated understanding of personality in non-human animals.