There are four stages in Piaget's theory of cognitive development:

1. Sensorimotor Stage (birth to 2 years): the individual "knows" through active interaction with environment. He/or she becomes aware of cause and effect relationships and learns that objects exist even when not in view. The child imitates crudely the action of others.

2. Preoperational Stage (ages 2 to 6 years): the individual begins by being very egocentric. Language and mental representations develop. Objects are classified on just one characteristic at a time.

3. Concrete Operations Stage (ages 7 to 12 years): the individual develops conservation of volume, length, etc., organizes objects into ordered categories, understands rational terms (i.e. bigger than or above), and begins using simple logic.

4. Formal Operations Stage (ages 12 and over): the individual's thinking becomes abstract and symbolic. His/or her reasoning skills and a sense of hypothetical concept develop.

These stages apply to all individuals and indicate a qualitative difference in: object permanence, egocentrism, conservation, abstract thinking, and centration.

The major concepts associated with Piaget's theory include:

1. Accommodation: the process of altering or revising an existing schema in light of new information.

2. Assimilation: the process of adding new material or information to an existing schema.

3. Object Permanence: the appreciation that an object no longer in view can still exist and reappear later (this occurs early during stage two).

4. Schema: a system of organized general knowledge stored in long-term memory that guides the encoding and retrieval of information.