More than 40% of the CEOs of the S&P 500 firms sit on outside boards, despite the

substantial opportunity cost of CEO time. We examine the potential benefit to home

firm of CEOs' outside directorship. External directorships generate economically signif-

icant performance improvements for the CEO's primary firm. However, these benefits

diminish when CEOs hold an excessive number of outside board positions. Our findings

support a learning mechanism where CEOs gain strategic insights through directorships

at firms in different industries, non-competitors, and supply chain partners, facilitated

by exposure to diverse business environments, unrestricted knowledge sharing, and im-

proved understanding of operational interdependencies. We also show that the strategic

importance of supply chain directorships can outweigh logistical barriers such as direct

flight availability. Our causal evidence from CEOs' first outside directorships suggests

that the positive learning effect is driven more by individual CEO attributes than by the

firm's initial experience with having a CEO on an external board.