Clinical research into the psychopathology of attention deficit/hyperactivity disorder (AD/HD) has established reliable measures of the defining behavioural problems: inattentiveness, overactivity, and impulsiveness. Experimental measures have defined them with increasing accuracy, but research is still needed before it is clear what processes need to be explained at the level of brain dysfunction. Understanding the associations of AD/HD, and the way they change over time, requires the approach of developmental psychopathology. This paper outlines some key concepts. The possible heterogeneity of AD/HD is stressed, with substantial differences being associated with the predominant type of symptom, the situations in which it is expressed, and the types of co-existing problems. Developmental influences on course need not be the same as initiating causes and a transactional model is outlined. Cultural and familial factors modulate the course of this biologically initiated disorder. Genetic influences appear to be strong and a combination of behavioural and molecular genetics with longitudinal study offers hope of advances in nosology.