It is widely agreed that the DSM-5, the handbook of psychiatric diagnosis, suffers from both high overlap among its putative disorders and high heterogeneity (variability) within each disorder. While these may appear to be opposite problems, in fact both may stem from failure to recognize transdiagnostic dimensions of emotion, cognition, and personality, among others, that inform psychopathology. These fundamental nosological challenges are exemplified in the case of attention-deficit/hyperactivity disorder (ADHD). In ADHD, broad clinical heterogeneity has defied easy clinical prediction of outcomes or clean statistical differentiation of meaningful, biologically informative sub-groups. Progress for ADHD heterogeneity looks promising, however, when we consider 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. Focusing on developments in the study of temperament traits and ADHD as they relate to emotional dysregulation, several lines of investigation are proving useful. Utilization of selective computational models, biological validators, and longitudinal analyses points toward potential improvements in nosology and clinical assessment in the future by taking temperament traits into account.