There is limited understanding of the problems associated with repeated neuropsychological assessment in children, including the statistics used to guide decisions about cognitive change. Further, clinicians rarely consider change in cognitive function when evaluating treatment response in individual children with ADHD. This is most likely due to a lack of suitable assessment tasks as well as clinicians’ limited awareness of the appropriate statistical techniques for measuring cognitive change in individuals. This book outlines a study investigating the application of a statistically principled decision rule to the cognitive and behavioural measures of individual children with ADHD in order to classify a significant, positive response to medication.