In recent years, an impressive volume of evidence has accumulated demonstrating that youth involvement in structured, organized activities (e.g. school sports, community clubs) may facilitate positive youth development. We present a theory-based framework for studying structured activity involvement (SAI) as a context for positive youth development based on two key dimensions: breadth and intensity of involvement. Our main goal is to demonstrate the separability, salience, and developmental significance of these two dimensions. We review three developmental theoretical approaches (identity development, life-span selection-optimization-compensation theory, and affordances) that support our conceptualization of breadth and intensity as salient and significant dimensions of SAI. We also summarize our recent program of research on SAI demonstrating the separability of breadth and intensity dimensions, which shows links between these dimensions and indicators of positive development. Finally, we discuss how the proposed breadth-intensity approach could be used to extend research on the linkage between youth SAI and successful development.