AbstractBackground. Research has established that people with ADHD have an in-creased risk of developing depression. Furthermore, this comorbidity is followed by increasing consequences on both the individual life quality and the societal burden and economics. Multiple explanations models and research on this relation exist. The ob-jective of this project was to explain and understand the etiology of comorbid ADHD and depression. Furthermore, the aim was to understand how this comorbidity could be treated and prevented Method. To answer this, the project was split in 3 sections, each contributing with some explanation. The first section presented different explana-tion models as well as their rationale and complexity. Furthermore, different research designs applicability in investigating causality were presented. The second part of the project investigated the evidence of direct and indirect causational factors as explana-tions of a relation from ADHD to depression, in a systematic review. The third section aimed to understand the best prevention and interventions strategies for depression in an ADHD population, based on the finding of the systematic review and the national clinical guidelines of Denmark as well as other research. Results In the first section it was established that both common etiology and direct and indirect causal factors of ADHD most likely contribute to the explanation of the association between ADHD and depression. Furthermore, the complexity of these processes was presented, demonstrating that these can be difficult to disentangle from each other in research. The second part found suggestions of a negative effect of having; the ADHD-IN subtype, decreasing self-perception, sleep problems, maternal psychopathology, traumatic expe-riences, and social difficulties, as well as no positive or negative effect from ADHD medication and performance EF. Lastly, mixed findings existed regarding the influence of ADHD symptoms, academic difficulties, and the family environment. The third sec-tion established that research is lacking on intervention and prevention strategies target-ing specifically comorbid ADHD and depression. However, multiple treatments al-ready suggested for ADHD or depression, combined well with the findings of the SR, suggesting a possible effect on depression. Examples of such includes, parent training programs, sleep interventions, cognitive behavioral therapy, and social skills training. In Conclusion the association between ADHD and depression seems to include a complex etiology. However, indirect, and direct factors of the ADHD diagnosis could explain part of this, and multiple factors is suggested in the research. Furthermore, these add well up with many intervention strategies already recommended in clinical guidelines.