Recent research suggests that people obtain complex, useful knowledge from other people with whom they work closely and frequently (i.e., strong ties). Yet there has been only limited systematic empirical work examining why strong ties are important for knowledge transfer. Based on a review of the social network, trust, and knowledge/organizational learning literatures, we propose a model whereby two-party (dyadic) trust mediates the relationship between strong ties and effective knowledge transfer. We tested this model with a two-stage survey in three companies in different countries and found strong support. First, the relationship between strong ties and effective knowledge transfer (as reported by the knowledge receiver) was mediated by competence- and benevolence-based trust. Second, once we controlled for these two dimensions of trust, it was actually weak ties that provided the most useful knowledge. This latter finding is consistent with prior research suggesting that weak ties provide access to n...