Although emotion has long been considered important to psychotherapeutic process, empirical assessment of its impact has emerged only recently. The present study applied two meta-analyses to explore the association between therapist expression of emotion and psychotherapy outcome, and client expression of emotion and psychotherapy outcome. Overall, 66 studies (13 for the therapist meta-analysis and 43 for the client meta-analysis) were included. A significant medium effect size was found between the therapist's emotional expression and outcomes (d = 0.56) and a significant medium-to-large effect size between the client's emotional expression and outcomes (d = 0.85). Third-party rating of emotional expression emerged as a significant moderator of outcomes. Limitations of the research, diversity considerations, and therapeutic practices that conclude the article are then presented. Clinical Impact Statement Question: We attempted to address the question of what the relationship between client and therapist expressions of affect on therapeutic outcomes was. Findings: Therapist expression of affect had a medium effect on client outcome, whereas client expression of affect had a medium-to-large effect on client outcome. Meaning: Clinicians may wish to reexamine the role of the expression of affect in the therapeutic relationship. Next Steps: Training to recognize and facilitate client emotional expression, as well as training focused on appropriate therapist emotional expression, would be warranted.