Nonsuicidal (NSSI) and suicidal self-injurious behaviors (SB) are common in adolescence. They occur in the general public but are particularly prevalent in psychiatric populations. The recognition of NSSI and SB as clinically relevant transdiagnostic phenomena has led to significant growth in scientific literature. However, profound knowledge regarding the courses and specific predictors is still scarce. The aim of the present dissertation is to address critical research questions with a focus on longitudinal analyses in a high-risk population of help-seeking adolescents. In the first study, the long-term effects of a brief intervention for adolescents with NSSI were examined. In a period of two to four years after the initial treatment phase, incidents of NSSI and SB reduced further, as did depression and borderline personality disorder criteria. Many patients received additional psychotherapy during the follow-up period which further decreased the frequency of NSSI. The focus of the second study was on the trajectory of NSSI over one year. An individualized analytical approach revealed that 75% of participants reduced the frequency of NSSI by at least half and 25% stopped NSSI completely. One in ten adolescents deteriorated over time and relapses were common. Predictors of change included depression, baseline NSSI frequency and the duration of inpatient treatment. The third publication analyzed the predictive value of self-rated risk for suicide attempts. Out of several clinically feasible variables, self-rated risk was the best predictor of actual suicide attempts. Depressive symptoms moderated the effect such that higher depression severity was linked to less accurate self-ratings. Suicidal and nonsuicidal self-injurious behaviors are complex and highly heterogeneous phenomena. Overall, the results reported in this dissertation provide important insight into the trajectory, treatment, and prediction of NSSI and SB in at-risk youth. Sophisticated longitudinal analyses with a focus on individual paths and alternative data sources have the potential to improve the current understanding of NSSI and SB and provide crucial knowledge for clinicians
