Conscientiousness is a personality construct that is a core determinant of health, positive aging, and human capital. A large body of work has contributed to our understanding of this important aspect of personality, but there are multiple conceptual and methodological issues that complicate our understanding of conscientiousness. Toward this end, we review (a) the conceptual standing of conscientiousness as a personality trait, (b) past research focusing on the underlying dimensions of conscientiousness, (c) the nomological network in which conscientiousness is embedded, and (d) the diverse methods that have been used to assess dimensions of conscientiousness. We conclude with recommendations for improving our understanding of the construct of conscientiousness, methods of assessment, and etiological underpinnings of conscientiousness. We believe this article can serve an important role in the larger goal of better understanding conscientiousness and its core role in the health of our society.