Depression is the leading mental health cause of disability in the world and has become more common in young people in the last few years. Young people with attention deficit/hyperactivity disorder (ADHD) are around five-times more likely to experience depression than those without ADHD. Depression in those with ADHD is especially serious – for example with a higher risk of suicide compared to either condition alone. However, standard interventions for depression do not seem to be as effective for young people with ADHD. To be able to advise future depression interventions, we need to understand the link between ADHD and depression. This project will examine (i) how the link between ADHD and depression develops across childhood, adolescence and young adulthood, (ii) cognitive and clinical factors that link ADHD and depression, and (iii) interpersonal relationship problems that link ADHD and depression.