Positive youth development (PYD) is an emerging area of study and practice that targets fostering the assets of young people to avoid problem behaviors and excel in meeting diverse life challenges. This paper describes how PYD evolved from treating problem behaviors to preventing problem behaviors in at-risk youth, to more recently helping all youth thrive and excel in numerous domains. Although evidence to inform community policy and practice has emerged, there is a lack of consensus on how to define PYD, and this lack of consensus has impacted progress in PYD physical activity behavioral science. This paper recommends PYD physical activity behavioral science reject disciplinary boundaries and (a) examine the nature of person-environment interaction in the context of physical activity as the primary outcome, (b) target big-picture physical activity outcome questions, and (c) come to a consensus on the domains of physical activity behavioral science research competencies.