© 2019 by The University of Chicago. All rights reserved. This paper considers cost-reducing R&D investment with spillovers in a Cournot oligopoly with overlapping ownership. We show that overlapping ownership leads to internalization of rivals’ profits by firms and find that, for demand not too convex, increases in overlapping ownership increase (decrease) R&D and output for high (low) enough spillovers while they increase R&D but decrease output for intermediate levels of spillovers. There is scope for overlapping ownership to improve welfare and consumer surplus, provided that spillovers are sufficiently large. The results obtained are robust when R&D has commitment value and in a Bertrand oligopoly model with product differentiation.