We explore the role of interfirm alliances as a mechanism for sharing technological knowledge. We argue that knowledge flows between alliance partners will be greater than flows between pairs of nonallied firms, and less than flows between units within single firms. Using patent citations as a proxy for knowledge flows, we find results that are consistent with these expectations. We then explore how firm characteristics affect knowledge flows within alliances and find positive effects due to technological, geographic, and business similarities between partners. We use alliance data from merit, patent data from the uspto, and firm data from compustat.