Conscientiousness is a fundamental aspect of human personality, one that is closely linked with various favorable life outcomes. Despite its importance in humans, conscientiousness has received little attention as to how it may have evolved, or whether it provides similar fitness benefits in other animals. To date, research in animal personality has found consistent support for the presence of all major dimensions of human personality in other animals except conscientiousness. In this review, we investigate conscientiousness at the level of traits and facets (clusters of closely related traits). A systematic review of the literature retrieved 876 relevant publications describing attributes of conscientiousness in other animal species. A factor analysis of citation counts revealed 2 major dimensions representing 9 distinct facets of conscientiousness in nonhumans. These facets, together with their underlying personality traits, exhibit individual variability, are generally known to be heritable, and often offer clear fitness benefits to individuals. Other facets of conscientiousness appear to be unique to humans. Publication biases, research biases, and anthropomorphism may all play a role in the structure of the evidence we report. We conclude by suggesting fruitful areas of future research to further elucidate the presence and functional roles of conscientiousness in animals.