Self-determination theory can be viewed as a theory of optimal relationship development and functioning. We examine the concept of self that is employed in self-determination theory and explain how its unique definition allows an important and novel characterization of investing one’s "self" in romantic relationships. A self-determined perspective on romantic relationships integrates several theories on romantic relationship development, but also goes beyond them by explicitly articulating the personality, developmental, and situational factors that facilitate optimal self-investment and relational functioning. Self-determination promotes openness rather than defensiveness and facilitates perspective-taking, authenticity, and support of close others. The dyadic context of romantic relationships affords great opportunity for theoretical development and integration of self-determination theory with current theories of interdependence and relational well-being.