<p>Sexual desire initially draws romantic partners together and continues to influence relationship functioning once relationships are more established. However, sexual interactions between romantic partners do not occur in a vacuum, and sexual desire can be influenced by the relational context as well as other situational factors. In this Review, we consider how sexual desire and relationship functioning influence each other across relationship development. First, we delineate how sexual desire changes over time and shapes relationship trajectories. We then introduce the relationship development model of sexual desire, which clarifies the functional significance of sexual desire in relationship development and the circumstances in which this function becomes particularly prominent. Next, we consider the reverse causal direction, providing an overview of how contextual, relational and individual factors influence couples’ sexual well-being and the inclination to seek sexual gratification outside the current relationship. We conclude by considering unanswered questions in the field and discuss how novel approaches, such as using dating apps to recruit people at the earliest stages of their relationship, might provide insight into these questions.</p>