We unpack the black box of open innovation, external knowledge search in particular, by showing how firms use not only R&D but also manufacturing and marketing for external knowledge sourcing for product and process innovation. Our econometric model reveals that the inverted U-shaped relationships found by Laursen and Salter (2006) for external search breadth hold for all functional areas but those for external search depth hold only for R&D-based search aiming at product innovation. In every case, we find that observed firms’ breadth suboptimal. We also find suboptimality for search depth: the optimal depth level is the maximum for marketing-based and manufacturing-based searches. We also specifically show that marketing is the dominant base for external sourcing for product innovation, and manufacturing is the dominant base for external sourcing for process innovation. Finally, we find strong complementarities among the roles of the three functional areas, which suggests that firms develop different absorptive capacities at the same time and do not substitute one for another.