The general aim of the thesis was to contribute to the knowledge about co-occurring symptoms of ADHD and depression in adolescence, focusing on sex differences, as well as aetiology, help-seeking and assessment. Studies I–III used epidemiological samples of self-reports from all students in Västmanland aged 15–16 and 17–18 years. Study I investigated the prevalence of co-occurring symptoms of ADHD and depression, as well as associations between co-occurring symptoms of ADHD and depression and one environmental stress factor; experience of sexual abuse. Study II examined associations between one biological factor—a polymorphism in TFAP-2β—and co-occurring symptoms of ADHD with andco-occurringsymptoms of depression. Study III investigated the association between the parent–adolescent relationship and seeking help from specialized mental health services in relation to symptoms of ADHD and/or depression. Study IV was a clinical study among adolescent psychiatric patients that compared self-reported ADHD symptoms via the Adult ADHD Self-Report Scale–Adolescent version (ASRS-A) and the Adult ADHD Self-Report Scale–Adolescent–Screening version (ASRS-A-S) with an ADHD diagnosis determined by the gold-standard method; the Kiddie Schedule of Affective Disorders and Schizophrenia diagnostic interview. Studies I–III showed that the phenotype of co-occurring symptoms of ADHD and depression is frequent, with a distinct preponderance among girls. Approximately 50% of both boys and girls with co-occurring symptoms of ADHD and depression had also experienced sexual abuse, indicating that this is a group with multiple risk factors for long-term impaired mental health. Results also support biological sex differences because girls with symptoms of ADHD and a common polymorphism of TFAP-2β (absence of a 9 repeat) reported more symptoms of depression, but boys did not. Further, only 5% of the adolescents with symptoms of ADHD and/or depression sought help from specialized mental health services. The co-occurrence of symptoms of ADHD and depression was a stronger predictor of help-seeking than all other psychosocial factors investigated, including secure attachment cognitions styles to parents. Among help-seeking girls, co-occurring symptoms of ADHD and depression were more common than symptoms of ADHD without co-occurring symptoms of depression. The ASRS-A/ASRS-A-S showed promising psychometric properties for further validation in adolescentsresults as a screening tool for use in adolescents.