Literature has identified formal and informal channels in university knowledge and technology transfer (KTT). While formal KTT typically involves a legal contract on a patent or on collaborative research activities, informal transfer channels refer to personal contacts and hence to the tacit dimension of knowledge transfer. Research is, however, scarce regarding the interaction of formal and informal transfer mechanisms. In this paper, we analyze whether these activities are mutually reinforcing, i.e., complementary. Our analysis is based on a comprehensive data-set of more than 2,000 German manufacturing firms and confirms a complementary relationship between formal and informal KTT modes: using both transfer channels contributes to higher innovation performance. The management of the firm should therefore strive to maintain close informal relationships with universities to realize the full potential of formal KTT.