Nonsuicidal self-injury (NSSI) was included as a condition for further study in the DSM-5. Therefore, it is necessary to investigate the suggested diagnostic criteria and the clinical and psychological correlates. In order to provide an optimal treatment best tailored to the patients need, a clear differentiation between Borderline Personality Disorder (BPD) and NSSI is needed. The investigation of personality traits specific to patients with NSSI might be helpful for this differentiation. Furthermore, social difficulties can often be a trigger for NSSI. However, little is known about how adolescents with NSSI perceive social situations. Therefore, we examined how adolescents with NSSI process emotional expressions. A new emotion recognition paradigm (ERP) using colored and morphed facial expressions of happiness, anger, sadness, disgust and fear was developed and evaluated in a student sample, selected for being high (HSA) or low socially anxious (LSA). HSA showed a tendency towards impaired emotion recognition, and the paradigm demonstrated good construct validity. For the main study, we investigated characteristics of NSSI, clinical and psychological correlates, personality traits and emotion recognition. We examined 57 adolescents with NSSI diagnosis, 12 adolescents with NSSI without impairment/distress and 14 adolescents with BPD, 32 clinical controls without NSSI, and 64 nonclinical controls. Participants were interviewed regarding mental disorders, filled out self-report questionnaires and participated in the ERP. Results indicate that adolescents with NSSI experienced a higher level of impairment than clinical controls. There were similarities between adolescents with NSSI and adolescents with BPD, but also important differences. Adolescents with NSSI were characterized by specific personality traits such as high harm avoidance and novelty seeking compared to clinical controls. In adolescents with BPD, these personality traits were even more pronounced. No group differences in the recognition of facial expressions were found. Nonetheless compared to the control group, adolescents with NSSI rated the stimuli as significantly more unpleasant and arousing. In conclusion, NSSI is a highly impairing disorder characterized by high comorbidity with various disorders and by specific personality traits, providing further evidence that NSSI should be handled as a distinct diagnostic entity. Consequently, the proposed DSM-5 diagnostic criteria for NSSI are useful and necessary.