Developing the understanding of ADHD heterogeneity appears promising when researchers consider dimensions of trait affectivity. These dimensions can include surgency (i.e., high levels of sociability, activity, and positive emotion) 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. To discuss this research, Dr. Teresa Treat, Professor of Psychology at University of Iowa, interviews Joel Nigg, Professor of Psychiatry and Behavioral Neuroscience at the Oregon Health and Science University.