Piaget's cognitive-developmental stage theory stands as one of three dominant twentieth-century positions on cognitive development. Piaget conceived of human cognition as a network of mental structures created by an active organism constantly striving to make sense of experience. Piaget believed that children move through four stages of development-sensorimotor, preoperational, concrete operational, and formal operational-during which the exploratory behaviors of infants are transformed into the abstract, logical intelligence of adolescence and adulthood. Piaget's stage sequence, which emphasizes that stages are invariant and universal, groups together similar qualitative changes in many schemes that occur during the same period of development.

According to Piaget, specific psychological structures, or schemes, change with age through the exercise of two important intellectual functions: adaptation and organization. Piaget described infants' special means of adapting their first schemes as the circular reaction-stumbling onto a new experience caused by the baby's own motor activity, and then trying to repeat the event. He considered revisions in the circular reaction so important that he named the six sensorimotor substages after them.

Some researchers believe that important schemes develop through perceptual means-by looking and listening-rather than just through acting on the world. Other researchers take a nativist view, believing that infants' built-in, core understandings become more elaborate as they come in contact with new information. A growing number of researchers have a modular view of the mind, viewing the mind as a collection of separate modules, or genetically prewired, special-purpose neural systems in the brain, which trigger new understandings with exposure to stimulation.

Recent research challenges many of Piaget's conclusions, often finding that infants and children develop cognitive capacities earlier than Piaget had believed. Today, virtually all experts agree that children's cognition is not as broadly stagelike as Piaget believed. Information-processing theorists believe that thought processes are similar at all ages-just present to a greater or lesser extent-and that uneven performance across tasks can largely be accounted for by variations in children's knowledge and experience. The neo-Piagetian perspective combines Piaget's stage approach with information-processing ideas.

Piaget's theory has had a major impact on education with its emphasis on discovery learning, sensitivity to children's readiness to learn, and acceptance of individual differences. Vygotsky's sociocultural theory, a second major theory of cognitive development, also views children as active seekers of knowledge, but rich social and cultural contexts profoundly affect the way their cognitive world is structured. Whereas Piaget concluded that young children's language is egocentric and nonsocial, Vygotsky reasoned that children speak to themselves for self-guidance and self-direction. Because language helps children think about their own behavior and select courses of action, Vygotsky regarded it as the foundation for all higher cognitive processes. Vygotsky believed that through joint activities with more mature members of society, children come to master activities and think in ways that have meaning in their culture. He believed that children learn best when tasks are in their zone of proximal development, a range of tasks that the child cannot yet handle alone but can accomplish with the help of adults and more skilled peers.

Vygotsky's theory has also influenced education through concepts and techniques such as assisted discovery, peer collaboration, reciprocal teaching, and cooperative learning. A new Vygotsky-inspired educational approach transforms classrooms into communities of learners, where no distinction is made between adult and child contributions; all collaborate and develop. An evaluation of Vygotsky's theory indicates that its emphasis on the role of language may not accurately describe cognitive development in all cultures. Also, by focusing on the cultural line of development, his theory does not describe exactly how elementary cognitive processes contribute to higher cognitive processes derived from social experience.