The term 'executive function' refers to a complex cognitive construct that encompasses the set of processes that underlie flexible goal-directed behaviour (e.g. planning, inhibitory control, attentional flexibility, working memory). For much of the last century, research into executive functions centred almost exclusively on adult populations. Three factors contributed significantly to this heavy focus upon adulthood. First, executive functions are closely associated with the pre-frontal cortex, an area of the brain that was long thought to become functionally mature very late in development, around adolescence (Golden, 1981; but see Rakic et al., 1986;Diamond, 1988Diamond, , 1991)). Second, primate studies and early research on head injuries suggested that the consequences of juvenile lesions to the prefrontal cortex did not become apparent until adulthood (the so-called 'Kennard effect'). Third, executive functions are crucial in situations that involve novelty, trouble-shooting, multiple constraints or ambiguity (Shallice and Burgess, 1991). Standard executive function tests were, therefore, designed to be difficult, and so are often developmentally inappropriate for use with children. Taken together, these three factors led to a chronic neglect of developmental changes in children's emerging executive functions. Why then does this topic warrant a special issue of Infant and Child Development?
In part, the remarkable increase of interest in early executive functions is tied to the growth of a new discipline over the past 20 years, that of developmental neuro-psychology. Indeed, impairments in executive functions are now thought to play a key role in a range of developmental disorders, including attentiondeficit hyperactivity disorder (ADHD) and autism (Ozonoff, 1997). Four papers in this special issue are devoted to these two important developmental disorders. Brophy, Taylor and Hughes report on a 3-year follow-up study of 'hard-tomanage' children at risk of ADHD who showed significant impairments in executive function as 4-year olds (Hughes et al., 1998). Perner, Kain and Barchfeld also examine the cognitive correlates of executive dysfunction in children 'at risk'