The ongoing COVID-19 pandemic represents a non-normative life event and is associated with various pandemic-related stressors. Research at the onset of the COVID-19 pandemic indicated an increase in psychological distress in the general population. However, over the course of the COVID-19 pandemic, findings increasingly showed that, on average, the general population appeared to recover from psychological distress. Nonetheless, a substantial minority appears to have become and remained psychologically distressed by the COVID-19 pandemic. Internet-based self-help interventions might be promising in the treatment of this COVID-19 related psychological distress. Against this background, the aim of this doctoral thesis is to investigate the efficacy of an internet-based self-help intervention for COVID-19 related psychological distress. In addition, by exploring predictors of treatment outcome, the aim is to identify who might benefit from such an intervention. For this purpose, three articles are presented. The first article presents the study protocol of the ROCO project, in which the efficacy of an internet-based self-help intervention for COVID-19 related psychological distress was evaluated in a randomized controlled trial (RCT). In the second article, the main results of this RCT are reported and in the third article, in a secondary analysis, predictors of treatment outcome were explored. Results indicate that the intervention was not effective in reducing depressive symptoms but led to a significant increase in emotion regulation skills and resilience. In addition, with regard to depressive symptoms, young adults in particular seem to benefit from the intervention, and with regard to resilience, individuals with higher pre-existing resources seem to particularly benefit. In this doctoral thesis, the articles are embedded in both the underlying theoretical framework and the current state of research. Moreover, results are summarized, discussed, and implications for coping with possible future pandemics are derived.