This chapter describes the self-expansion model as a conceptual framework for understanding romantic love and relationship well-being. The self-expansion model incorporates two main principles: the motivational principle and the inclusion of others in the self principle. The motivational principle proposes that self-expansion is a basic human motive to grow the self, similar to what other theorist’s term exploration, curiosity, competence, or self-development. The inclusion of others in the self principle suggests that people expand themselves through close relationships, by including the other's resources, knowledge, perspectives, identities, social networks, etc. in one's self-concept. Self-expansion might be especially important in lasting relationships as couples try to maintain love, excitement, passion, and intimacy. Couples can try to maximize self-expansion benefits by seeking out highly rewarding experiences that both partners enjoy, as they are more likely to want to engage in these repeatedly and thus have lasting effects.