Attention-deficit/hyperactivity disorder (ADHD) is common, is costly, and confers substantial risk of chronic poor outcome. It exemplifies the need to better understand variation within psychiatric conditions (heterogeneity) and co-occurrence of different conditions. In particular, ADHD’s broad clinical heterogeneity has required clarification, as subtypes for which there is consensus have not emerged. The outlook for making progress in understanding ADHD heterogeneity is promising, however, when one considers dimensions of trait affectivity, such as surgency and negative affectivity; their constituent lower-order traits, such as irritability; and the integrative function of self-regulation. Several lines of investigation focusing on ADHD and temperament traits (related to emotional regulation and dysregulation) are proving useful. Work in this area has the potential to improve theory, nosology, and clinical assessment in the future.