Suppliers play an increasingly central role in helping firms achieve their new product development (NPD) goals. The literature implicitly assumes that suppliers are able to meet or exceed the quality standards and technological expectations of the firm, and yet, in practice, suppliers often lack the technological capabilities needed to undertake collaborative NPD. In such situations, a firm may choose to intervene and actively develop the supplier's technological and product development capabilities. We develop a theoretical framework that conceptualizes supplier development activities within interorganizational NPD projects as part of a bilateral knowledge-sharing process: design recommendations, technical specifications, and new technology flow from supplier to the firm, and in turn, the firm can implement supplier development activities to upgrade the supplier's technological capabilities. Antecedents (supplier responsibility, skills similarity, single sourcing strategy) and consequences of supplier development activities (on supplier, product, and project performance) are examined using a sample of 153 interorganizational NPD projects within UK manufacturers. We find broad support for our hypotheses. In particular, we show that the relational rents (in the form of improved product and project performance) attained from supplier development activities in new product development are not achieved directly, but rather indirectly, via improvements in the supplier's creative and technological capabilities. Our results emphasize the importance of adopting a strategic view of the potential returns available from investing in the NPD capabilities of key suppliers, and provide clues about underlying reasons for the suboptimal experiences of many companies' collaborative NPD projects.