We develop and examine a framework for the knowledge spillover theory of entrepreneurship in alliances. Our framework regards entrepreneurial opportunity as endogenous, and entrepreneurial firms as heterogeneous. The empirical findings show that partners' knowledge protection, which is regarded as a knowledge filter, can increase knowledge spillovers in an alliance. Moreover, this relationship is contingent on the strength of a focal firm's entrepreneurial orientation and on alliance type (equity joint venture versus nonequity joint venture). Results also reveal that knowledge spillovers in an alliance enhance alliance performance more significantly than they enhance firm performance.