ADHD (attention deficit hyperactivity disorder) is increasingly viewed as a neurodevelopmental disorder. Complex findings have emerged regarding ADHD etiology and pathophysiology. Part of this complexity is due to insufficient characterization of both clinical and mechanistic heterogeneity among children (or adults) who meet ADHD criteria. This presentation will consider some evidence in support of the centrality of this heterogeneity problem in ADHD. It will then consider the potential role of gene x environment interactions and of environmental effects on ADHD, a highly heritable condition.