ADHD is the most common neurodevelopmental disorder, affecting around 5% of children. It is highly variable in how it presents, and often symptoms from other disorders, such as autism spectrum disorders, are present in ADHD. Improving diagnosis and treatment of ADHD, understanding its diversity and how it co-occurs with other disorders, requires that we re-think how we currently classify disorders, and instead move towards objective, data-driven identification of clusters of risk traits.