This study examines the factors (trust, communication, supplier relationship specific adaptations, supplier flexibility, and relationship history) that influence knowledge transfer from the supplier to the customer firm in new product development, and the impact of knowledge transfer on product development performance. It also suggests that knowledge tacitness moderates these relationships. Based on a sample of 186 US firms, this study finds that trust, communication, supplier relationship specific adaptations, and supplier flexibility influence knowledge transfer. Knowledge transfer, in turn, has an effect on new product development performance. Mixed findings have also been reported in this paper with respect to the moderating effects of knowledge tacitness. Trust, supplier flexibility, and relationship history are more important for tacit knowledge transfer than for explicit knowledge. Knowledge tacitness does not moderate the relationship between knowledge transfer and NPD performance. However, the results show that both tacit and explicit knowledge transfer significantly affects NPD performance.