A firm's inventors are repositories of its core expertise that constitutes its competitive advantage. This knowledge is subject to erosion when an inventor exits the firm. Little is known, however, about what makes an inventor with core knowledge susceptible to exit. We develop a model of exit in which inventor knowledge may be core, unique, and complex, which determines the likelihood of her or his exit from a firm. We study inventor exit from IBM using a long panel of USPTO data (1975–2010) and find that an inventor with core knowledge is more likely to exit from IBM when she or he has more unique and less complex knowledge. These factors also determine whether the inventor subsequently joins a rival firm or a non–rival firm.