Connections between youth activity involvement and indicators of successful development were examined in a longitudinal high school sample. Drawing on theories of expertise skill development (e.g., J. Côté, 1999); the selection, optimization, and compensation framework (P. B. Baltes, 1997); and theories of positive youth development (e.g., R. M. Lerner, J. B. Almerigi, C. Theokas, & J. Lerner, 2005), reciprocal associations between breadth and intensity of activity involvement and developmental success were explored. Time 1 breadth (but not intensity) and increases in breadth predicted higher levels of successful development at Time 2 (20 months later). Time 1 developmental success and improvements predicted greater Time 2 breadth and intensity. Implications for research and theory related to connections between youth activity involvement and successful development are discussed.