We present a leverage theory of reputation building with co-branding. We show that under certain conditions, co-branding that links unknown firms in a new sector with established firms in a mature sector allows the unknown firms to signal a high product quality and establish their own reputation. We compare this situation with a benchmark in which both sectors are new and firms signal their quality only with prices. We investigate how this comparison is affected by the nature of the technology linking the two sectors and a cross-sector inference problem that consumers might face in identifying the true cause of product failure. We find that co-branding facilitates the process in which a Þrm in the new sector to signal its product quality only if the co-branding sectors produce complementary inputs and consumers face a cross-sector inference problem. We apply our insight to economics of superstars, multinational firms and co-authorship.