Five studies develop and validate the Self- and Other-Interest Inventory, an individual-difference measure of the motivation to act in one’s own interest and the motivation to act in another’s interest that measures these motivations at the level of self-beliefs. Study 1 demonstrates that self- and other-interest can be measured reliably and validly, as independent constructs, with a self-report measure. Study 2 develops a version of the Self- and Other-Interest Inventory for use with a general population and demonstrates systematic changes in the relation between self- and other-interest scores with age. Study 3 shows that self- and other-interest scores vary independently, as a function of the accessibility of related values. Study 4 provides evidence that self-interest scores predict behaviors that benefit the self and that other-interest scores predict behaviors that benefit another person. Finally, Study 5 demonstrates that in situations that involve a trade-off between the pursuit of self-interest and the pursuit of other-interest, such as the prisoner’s dilemma, self- and other-interest scores contribute independently to behavioral prediction.