Given their backgrounds in classical ethology and in comparative psychology, researchers who study animal personality in biology and psychology, respectively, differ in how they measure personality, what questions they see as important, and how they address these questions. Despite these differences, both comparative psychologists and biologists embrace personality traits. By doing so, they have solved empirical and conceptual problems in animal behavior. Studies of animal personality have provided answers to questions about the evolution of human personality and have presented conceptual and empirical anomalies for sociocognitive theories. Animal personality research does not break from trait theories of personality. Instead, it enriches trait theories by conceiving of traits as not belonging to a species, but as expressed, with some modifications, across species. Broadening trait theory in this way has the potential to further enhance its ability to answer questions related to animal and human personality.