To answer the question of when are assets complementary, we investigate specific resource combinations along the value chain, focusing on two mechanisms that are central to combining resources for innovation in the pharmaceutical industry: recruitment and retention of star scientists, and 2) engagement in strategic alliances. We propose that resource combinations that focus on the same parts of the value chain are substitutes due to knowledge redundancies. Conversely, we hypothesize that resource combinations that link different parts of the value chain are complements due to integration of nonredundant knowledge. To test these hypotheses, we empirically track the innovative performance of 108 global pharmaceutical firms over three decades (1974–2003). Copyright © 2011 John Wiley & Sons, Ltd.