Behavioral and cognitive development is considered as ordered change within an individual during his/her lifespan, and not as sets of individual differences between persons or progression in stages. The concept 'developmental task' is elaborated in such a way as to stress situations that interacting individuals create and meet with, and which elicit transitions in behavior and, by implication, in the self. The concept is compared and proved to be compatible with the dynamic concepts of Russian activity theory and of the Galperinian concept of meaningful learning. The idea of a 'developmental task' and its dynamic characteristics is tentatively, but fruitfully, applied to different research topics, such as age-related crises, acquisition of a moral status, achieving educational independence, assessment of readiness for school, acquiring peer status, the acquisition of standards of morality in relation to self-image and to the moral atmosphere of the school or profession, and the experience of ageing. The book is intended for psychologists, educators and advanced students.