The present authors investigated whether an individual's motivations that are related to need fulfillment and self-expansion within a romantic relationship can predict self-reported susceptibility to infidelity. A sample of 109 college students (50 men, 59 women) who were in dating relationships completed questionnaires that assessed 5 types of variables of need fulfillment (i.e., intimacy, companionship, sex, security, and emotional involvement), 3 types of self-expansion variables (i.e., self-expansion, inclusion of the other in the self, and potential for self-expansion), and susceptibility to infidelity. As the present authors predicted, both sets of predictors (need fulfillment and self-expansion) significantly contributed to the variance in susceptibility to infidelity. The present findings indicated the possibility that, when a relationship is not able to fulfill needs or provide ample self-expansion for an individual, his or her susceptibility to infidelity increases.