Despite considerable interest in understanding how negative emotional expressions affect close relationships, there has been scant attention to the function of disappointment. The present dissertation aims to address the gap in the literature and provide a first examination of disappointment in romantic relationships. Using qualitative free recall, quantitative self-report, and daily-diary methodologies, Chapter 2 examines the frequency of negative emotional experiences in dating individuals, and confirms that disappointment is a frequently experienced negative emotion in romantic relationships. Participants report experiencing disappointment as frequently as anger, sadness, and jealousy, which are arguably the most often studied emotions in close relationships. Therefore, studying disappointment in the context of romantic relationships possesses theoretical significance in understanding negative emotional experiences in romantic relationships. Using diverse methodologies and large samples of married participants, Chapter 3 provides a first look at the function of perceiving disappointment from romantic partners. In an exploratory phase, participants described a past event wherein their partner expressed disappointment, as well as their emotional reactions and construals of their partner’s intentions and their motivation to change. Most participants reported that they experienced negative emotions in response to their partner’s disappointment. However, they also reported benign construals of their partner’s intentions and experiencing relationship-promoting motivations. In a theory-confirmation phase, three experiments provided strong causal evidence that perceiving partner’s disappointment boosts relationship-promoting motivations, compared to reflecting on an ordinary interaction with one’s partner, and compared to perceiving partner’s anger. Chapter 4 complements the findings in Chapter 3 by investigating the role of individual differences in chronic relationship quality on the downstream consequences of perceiving partner’s disappointment. Participants with high-quality relationships are more likely to generate benign construals of partner’s intentions, which in turn, experience enhanced relationship-promoting motivations. Finally, Chapter 5 discusses the social-functional implications of disappointment and synthesizes potential future research.