In this special section, 9 studies and 6 commentaries make a unique contribution to the study of personality. They focus on the five-factor model and, in particular, one of those 5: conscientiousness. This trait has had astonishing success in the actuarial prediction of adaptive outcomes in adulthood and aging, but we have little understanding of the mechanisms that account for this actuarial success. The current studies and comments marshal current knowledge of conscientiousness to advance a mechanistic understanding of these predictions and to exploit that understanding toward interventions to enhance robust adult development and healthy aging. In this introductory article, we underscore the strategy we used to invite presentations and commentary. First, we sought a clearer definition of conscientiousness and a review of its assessment. Second, we sought a review of how the components of this complex trait develop in childhood and are assembled across development. Third, we sought an understanding of how mechanisms linking conscientiousness and health might be transformed across the life span. Fourth, we scrutinized naturally occurring factors that moderate the links between conscientiousness and health for clues to successful interventions. Finally, we sought ways to pull these analyses together to outline the framework for a program of interventions that, collectively, might be applicable at specific points across the life span. Six commentaries place this project in sharp relief. They remind us that the causal status of the associations between conscientiousness and health, reported throughout our 9 studies, are uncertain at best. Second, they remind us that the concept of conscientiousness is still too spare: It fails to embody the social skills required for conscientious behavior, the moral judgment of self or other implicit in its assessment, or the neurobiological mechanisms that might account for differences among individuals. Third, they raise a potent counterfactual: What, in a practical sense, does conceptualization or assessment of conscientiousness contribute--if anything--to the design of interventions to enhance conscientious behavior?