The paper “Product innovation and the complementarities of external interfaces” by Sverre J. Herstad and myself is accepted for publication with the European Management Review. This paper investigates the relationship between new product introduction, and three constructs (search, collaboration and external R&D) developed to capture the different means by which firms link internal R&D to external inputs. By including interaction effects and applying detailed marginal effects analysis, it sheds new light on a research question, which has generated much empirical ambiguity: are external interfaces complements? Search diversity and collaboration diversity measure the extent to which different types of information sources and collaboration partners are used. Both affect innovation performance positively, and are complementary to each other. External R&D measures the relative importance of contract R&D, and is found to have a conditional negative impact which is reduced by search and reinforced when combined with collaboration. When including interaction effects involving the overall R&D intensity of firms, our findings suggest the existence of two competing ideal types of open innovation strategy and organization.