Severe irritability, characterized by an elevated disposition towards anger and provocation relative to peers, is highly impairing, disruptive and a common reason for referral to mental health services. It also co-occurs with several different mental health and neurodevelopmental conditions, such as ADHD, autism, anxiety, and depression. Importantly, severe irritability is currently classified differently in European (ICD-11) and American (DSM-5) diagnostic manuals, either as a behavioural or mood problem, respectively. A high prevalence of irritability in children with ADHD, along with genetic overlap, has also led to the hypothesis that irritability may be a core feature of ADHD, a neurodevelopmental condition. Recent research suggests that irritability may be different (e.g., more related to ADHD or mood/depression phenotypes) depending on the age when symptoms commence. Research that spans different ages is therefore very important. This project aims to test the hypothesis that childhood-onset persistent irritability, resembles ADHD in its developmental course and aetiology.