We use contingency theory to examine, for the first time, the incidence and effectiveness of CEO/COO duos. We argue that industry dynamism, extraordinary organizational task demands, and the CEO's own professional limitations will influence the decision to have a COO, as well as its effect on performance. Based on a large 10-year sample, we find some support for the contingency view in explaining the presence of COOs; we particularly find that CEOs who lack experience in operational activities and in managing the focal firm are relatively likely to have COOs. We find, however, essentially no support for the contingency view in explaining when COOs are most beneficial. Instead, we find strong evidence of a very substantial negative main effect: CEOs who have COOs deliver lower organizational performance than those who do not. Copyright © 2004 John Wiley & Sons, Ltd.