Does CEO overconfidence help to explain merger decisions? Overconfident CEOs over-estimate their ability to generate returns. As a result, they overpay for target companies and undertake value-destroying mergers. The effects are strongest if they have access to internal financing. We test these predictions using two proxies for overconfidence: CEOs’ personal over-investment in their company and their press portrayal. We find that the odds of making an acquisition are 65% higher if the CEO is classified as overconfident. The effect is largest if the merger is diversifying and does not require external financing. The market reaction at merger announcement ( - 90 basis points) is significantly more negative than for non-overconfident CEOs ( - 12 basis points). We consider alternative interpretations including inside information, signaling, and risk tolerance.