Research has linked youth activity involvement to positive development. However, past studies have confounded at least two separable dimensions of involvement: breadth (number of activities) and intensity (participation frequency). Theory and the limited available evidence suggest that these dimensions may make independent contributions to development. Based on self-reports from 7430 high school students, this study assessed whether breadth and intensity dimensions were related to each other, to a typical aggregate measure of involvement, and to various indicators of positive development. Breadth and intensity were moderately interrelated and, in combination, they explained the majority of the variance in the typical involvement measure. Both dimensions were positively related to each development index. When examined simultaneously, only breadth had a unique relation with each developmental index. Further, evidence of nonlinear effects was found. Findings were consistent across age levels. Implications for measurement of involvement and interpretations of the extant research are discussed.