Leading activity is a concept used within the tradition of Cultural-Historical Activity Theory (CHAT), to describe the activity, or cooperative human action, which plays the essential role in child development during a given developmental period. Although many activities may play a role in a child's development at any given time, the leading activity is theorized to be the type of social interaction that is most beneficial in terms of producing major developmental accomplishments, and preparing the child for the next period of development. Through engaging in leading activities, a child develops a wide range of capabilities, including emotional connection with others, motivation to engage in more complex social activities, the creation of new cognitive abilities, and the restructuring of old ones (Bodrova & Leong 2007: 98). Leading activity is a concept used within the tradition of Cultural-Historical Activity Theory (CHAT), to describe the activity, or cooperative human action, which plays the essential role in child development during a given developmental period. Although many activities may play a role in a child's development at any given time, the leading activity is theorized to be the type of social interaction that is most beneficial in terms of producing major developmental accomplishments, and preparing the child for the next period of development. Through engaging in leading activities, a child develops a wide range of capabilities, including emotional connection with others, motivation to engage in more complex social activities, the creation of new cognitive abilities, and the restructuring of old ones (Bodrova & Leong 2007: 98). The term 'leading activity' was first used by Lev Vygotsky(1967: 15-17) in describing sociodramatic play as the leading activity and source of development of preschoolers, but it was not systematically incorporated into Vygotsky's theory of child development. Later, however, Alexei Leontiev and other 'neo-Vygotskians' such as Alexander Zaporozhets and Daniel Elkonin (Zaporozhets 1997; Zaporozhets & Elkonin 1971) made the concept a fundamental element of their activity theory of child development. The concept has now been extended to several stages or periods in human development. The notion of a leading activity is part of a broader theory of activity that attempts to integrate cognitive, motivational, and social aspects of development. Despite many detailed descriptive accounts of the developmental forms of memory, perception, and cognition in various phases of childhood (e.g. Piaget's work), often missing is an explanation for how or why the child develops these psychological processes (Karpov 2003: 138). The exploration of leading activities seeks to illuminate these questions. Rather than biological maturation or stimulus-response learning, specific types of social activity are seen as generating human development. Because of its attention to causal dynamics, the neo-Vygotskian theory has been called 'the most comprehensive approach to the problem of determinants and mechanisms of child development (Karpov 2003: 138).' A leading activity is conceptualized as joint, social action with adults and/or peers that is oriented toward the external world. In the course of the leading activity, children develop new mental processes and motivations, which 'outgrow' their current activity and provide the basis for transition to a new leading activity (Kozulin, Gindis, Ageyev, & Miller 2003: 7). In most cases, the emergence of an activity can be seen long before it becomes the leading activity in a child's life (Bodrova & Leong 2007: 99). Adults and more capable peers who instruct or assist children in engaging in the leading activity are said to be providing mediation of the activity, and creating a zone of proximal development, which allows children to perform activities at a higher level than they could perform independently. Mediation of leading activities is theorized to help children develop by acquiring the use of cultural or psychological 'tools,' which transform children's mental processes. The activity considered leading for any given age or period of development depends on the type of society in which a child develops, and on the particular historical and cultural expectations for children of that age (Bodrova & Leong 2007). Neo-Vygotskians have proposed a sequence of children's optimal leading activities in modern industrialized societies (though leading activities may differ in agrarian or hunter-gatherer societies). Of course, not all children within a particular culture will engage equally in leading activities (or in equal 'quality' of activities), which leads to different developmental trajectories and outcomes. For example, children who lack opportunities to engage in rich, well-developed sociodramatic play during their preschool years appear to have greater difficulty with self-regulation and impulse control, characteristics associated with the diagnosis of Attention Deficit Hyperactivity Disorder (ADHD) (Bodrova & Leong 2007: 99)