In this chapter we describe the self-expansion model as a conceptual framework for understanding romantic love and relationship well-being. The self-expansion model (Aron & Aron, 1986) was developed to address basic processes underlying close relationships and, since, has been extended to other contexts. The self-expansion model incorporates two main principles: (1) the motivational principle proposes that self-expansion is a basic human motive to grow the self, similar to what other theorists term exploration, curiosity, competence, or self-development; (2) 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.