Until recently, attention deficit hyperactivity disorder (ADHD) was thought of as a childhood disorder that mostly affected boys; thus the majority of ADHD research neglects ADHD among girls, and to an even greater extent, ADHD among adult women. While in childhood, ADHD has a strong male preponderance (10:1 to 2:1 male:female ratio), by adulthood this imbalance has disappeared and women make up 50% of the adult ADHD population. One explanation for this change could be that girls with ADHD are missed in childhood and only join the ADHD population in adulthood when they are asked to report their own symptoms, rather than relying on parent or teacher report. This is supported by research that finds teachers are less likely to identify ADHD symptoms among girls, and that girls are more likely to present with symptoms of inattention than hyperactivity/impulsivity, which may cause them to ‘fly under the radar’ of parents and teachers. Alternatively, some girls may not the exhibit the full disorder in childhood, but their symptoms may be exacerbated by the challenges of adolescence and young adulthood, such that they begin to meet full ADHD criteria in adulthood. To better understand what characterizes these cases of missed/subthreshold ADHD among girls, this proposal seeks to identify childhood risk factors among girls across socioenvironmental, behavioural and cognitive domains that predict later-emerging adult ADHD.