Positive emotions broaden our thinking and help us build resources (Fredrickson, 1998, 2001), and different specific positive emotions appear to have unique adaptive functions (e.g. Stellar et al., 2017). Yet, clinical science has typically focused on the reduction of negative emotions in treatment (Carl et al., 2013). Expanding the study of positive emotions, especially specific ones like compassion and gratitude, in relationship and clinical science can reveal new processes that support healthy relationships and improve individual well-being (Repetti & McNeil, 2018).This dissertation asked how positive emotions are linked to relationship functioning and mental health, by investigating daily expressions of compassion and gratitude, and increases in positive emotions, in the contexts of marriage and psychotherapy. The question is addressed in three studies which encompass two samples of married couples and one clinical sample, cross-sectional and longitudinal designs, and observational and participant-report methodologies. Using intensive repeated measures collected over 5 weekdays, Study 1 found evidence of daily synchronization of spouses’ expressions of gratitude, and compassion synchrony was moderated by marital quality. Compassion may require more effort to express than gratitude and therefore its reciprocation is only characteristic of higher quality marriages. In Study 2, naturalistic observations of couples from the Center on Everyday Lives of Families (CELF) were coded for expressions of everyday compassion. Couples offered compassion frequently, on average twice per hour. Husbands who perceived their marriages as lower quality offered more compassion; wives offered more compassion when their perceived marital quality was high but their spouses’ was low. Higher rates of compassion were also expressed by both husbands and wives in couples where wives reported more depressive symptoms and husbands reported more neuroticism. Personal distress appeared to be associated with more frequent expressions of marital compassion. Study 3 examined the trajectories of positive and negative emotions as well as treatment outcomes over the course of psychotherapy in a psychology training clinic. Both increases in positive emotions and decreases in negative emotions independently predicted concurrent declines in symptom distress and improvement in relationship functioning, suggesting both are indicators of therapeutic change. However, changes in positive emotions were also predictive of future improvements in symptoms and relationship functioning, suggesting they may be precursors to therapeutic change.