We use detailed assessments of CEO personalities to explore the option-based measure of CEO overconfidence, Longholder, introduced by Malmendier and Tate (2005a) and widely used in the behavioral corporate finance and economics literatures. Longholder is significantly related to several specific characteristics and is negatively related to general ability. These relations also hold for overconfidence measures derived from CEOs’ earnings guidance. Investment-cash flow sensitivities are larger for both Longholder and less able CEOs. Overall, Longholder CEOs have many of the same characteristics traditionally associated with overconfident individuals, including lower general ability, supporting the interpretation of this measure as reflecting overconfidence. Institutional subscribers to the NBER working paper series, and residents of developing countries may download this paper without additional charge at www.nber.org.