A hallmark feature of individuals with borderline personality disorder (BPD) and social anxiety disorder (SAD) are severe interpersonal problems. According to cognitive theories, interpersonal problems are caused by biases in the processing of social information. Indeed, previous research has shown that individuals with BPD and SAD tend to process social information in a biased manner. For example, compared to individuals without a mental disorder, individuals with BPD evaluate facial expressions more negatively, and individuals with SAD direct their attention faster towards negative facial expressions. However, little is known about the processing of positive social information in BPD and SAD, and no study has compared the processing of positive social information between both disorders so far. The aim of this thesis is to unpack biases in the processing of positive social information in BPD and SAD in order to understand the nature of their interpersonal problems in more detail. To this end, the three studies presented in this thesis compared the processing of two specific types of positive social information – inclusive social situations and positive social feedback – between individuals with BPD and SAD. In the long term, these insights on shared as well as disorder-specific biases will help to treat interpersonal problems in BPD and SAD more effectively. Study 1 and study 2 examined whether individuals with BPD and SAD process inclusive social situations in a biased manner. Inclusive situations were operationalized as social inclusion (being equally included into a group) and social overinclusion (being overly included into a group). Structured clinical interviews were conducted to compare the processing of social inclusion and overinclusion among three groups: Participants with BPD (n = 29), participants with SAD (n = 28) and healthy controls (HCs; n = 28). All 85 participants played two rounds of the well-established Cyberball paradigm, an online ball-tossing game. In the first round, participants received the ball as often as their virtual co-players did (social inclusion); in the second round, participants received the ball more often than the co-players did (social overinclusion). In addition to self-report data, electroencephalography data was assessed. This enabled the examination of event-related potentials associated with the processing of inclusive social situations. Event-related potentials of interest were the P2 amplitude, an indicator for reward processing, and the P3 amplitude, an indicator for expectancy violation. Results of study 1 showed an enhanced P3 amplitude in participants with BPD and SAD irrespective of the condition (inclusion and overinclusion). This indicates that individuals with BPD and SAD tend to expect exclusion even if they find themselves in inclusive social situations. Self-report data revealed that participants with BPD and SAD emotionally responded more negatively to social inclusion, while their emotional response to social overinclusion mostly did not differ from HCs. Results of study 2 showed that the transition from social inclusion to social overinclusion was accompanied by an enhanced P2 amplitude in participants with BPD and HCs, but not in participants with SAD. This indicates that an increase in the level of social inclusion is perceived as rewarding by individuals with BPD and individuals without a mental disorder, but not by individuals with SAD. However, the increase in the level of social inclusion did not affect self-reported positive emotions and it is necessary to further clarify the association between the P2 amplitude and reward processing. Study 3 examined whether individuals with BPD and SAD fear positive social feedback. One hundred participants (three groups: 36 participants with BPD, 29 participants with SAD, 35 HCs) took part in an online assessment. Results of the Fear of Positive Evaluation Scale showed that individuals with BPD and individuals with SAD highly fear positive feedback. A hierarchical regression analysis further indicated that social anxiety explains high fear of positive feedback in BPD and SAD. In summary, the findings of this thesis point towards shared as well as disorder-specific biases in the processing of positive social information in BPD and SAD. On the one hand, individuals with BPD and individuals with SAD seem to process inclusive social situations in a biased manner. That is, they expect to be excluded even in inclusive social situations and emotionally respond to social inclusion more negatively. Furthermore, individuals with SAD and individuals with BPD seem to be afraid of positive social feedback. On the other hand, findings indicate that specifically individuals with SAD might not benefit from an increase in the level of social inclusion. On a general level, the findings of this thesis indicate that both disorders are characterized by negative social expectations (i.e., the expectation to be excluded) and maladaptive emotions (i.e., fear of positive feedback) even in the processing of positive social information. This knowledge is important for clinical practice. For a start, therapists should be aware that their patients tend to process positive social information in a biased manner, which might contribute to their interpersonal problems.