Behavioral and cognitive development is considered here as an ordered change in an individual throughout his or her lifespan, and not as sets of individual differences between persons, nor as stage-like progressions. The concept of developmental task is introduced, stressing contexts within which individuals meet, eliciting transitions in their behavior and, by implication, in the self. The developmental task concept is compatible with the activity theory of Gal'perin, especially the concept of meaningful learning. The authors show how their concept may be applied to age-related crises, the acquisition of a moral status, the achievement of educational independence, the assessment of readiness for school, acquisition of peer status, acquisition of concepts of morality, and the task of ageing. For professional psychologists and educationalists, and advanced research students in the same subjects.