This study aimed to address the following questions regarding the emotional experience of Dialectical Behavior Therapy clients with Borderline Personality Disorder: 1) How do positive and negative emotions change in therapy? 2) Does the severity of clients’ symptoms relate to affect? 3) Is affect related to clients’ perceptions of therapeutic alliance? 4) How are clients’ and therapists’ affect related? To test these questions, positive and negative affect ratings were collected from clients (N=77) and therapists (N=25) at the start and end of session. These ratings were tested in relation to alliance and severity ratings using Hierarchical Linear Modeling. Results indicated that clients’ positive affect increased while negative affect decreased from the start to the end of session. This pattern was mirrored over the course of treatment, but only the increase in positive affect was statistically significant over that time period. Severity was significantly related to affect, but in an unexpected direction (higher ratings of emotion dysregulation were associated with slight decreases in negative emotion and emotion lability). Additionally, clients’ positive emotion significantly predicted therapeutic alliance ratings, and therapist positive affect was significantly, positively related to clients’ positive emotion. These results indicate that client affect appears to change in treatment and may be related to severity, alliance, and therapist affect. Further exploration is needed to clarify these complex relationships given the differences between positive and negative affect and the surprising direction of the association between negative affect and emotion dysregulation.