Piaget defined four stages of cognitive development: sensorimotor, preoperational, concrete operational, and formal operational. Piaget developed his theory while observing his own nephew and daughter. Piaget's theory is unique in that, before this time, children were treated just like tiny adults in terms of what they could be taught. Piaget's theory laid the groundwork for age-appropriate education and skill development in children.

In the sensorimotor stage, which lasts from birth to approximately two years of age, children learn that they can manipulate their environment. Children learn by tasting, touching, hearing, and looking. Children also develop object permanence, which means that they know an object continues to exist even if they can no longer see it.

In the preoperational stage, which lasts from two to seven years of age, children can be egocentric and sometimes feel little empathy. They can also use words and pictures to represent objects. This helps the child better explain themselves when talking about objects. They still struggle with abstract thinking.

In the concrete operational stage, which lasts from seven to eleven years of age, children begin to develop inductive logic. They also understand the concept of conservation—for example, a tall skinny cup can hold as much as a short fat cup, provided that they can hold the same amount of ounces. They also understand that their thoughts are unique to them and that their viewpoints are unique.

In the formal operational stage, which takes place from the age of twelve, children develop abstract thought and deductive logic. Children can also think clearly about hypothetical situations. Teenagers can also grasp political and moral questions more easily and debate them with others.

Piaget's theory was revolutionary in that he pointed out that children changed the way they thought and that their continued development was not only attached to the continued acquisition of knowledge. This was important because it implied that the brain was still developing in children all the way through what we would call adolescence today.

Piaget argued that cognitive development occurred in four distinct stages. The first was a sensory motor stage, which occurred in the first two years of life. In this stage, babies learn through the consequences their physical actions, which include grabbing, sucking, eating, and so on. The second stage is what Piaget called a "preoperational" or intuitive phase. During this phase, the use of language allows a child to interact with the world, and thus to learn, in a variety of ways. But children at this stage are still essentially self-centered in that they can only really understand the world as they interact with it; they are incapable of empathy. The third phase is a "concrete operational" phase, and it lasted from age seven to about eleven. During this phase, though again, children were centered on their own world, and their own actions, they became aware of the ways their actions are organized into categories. In other words, they start to understand abstract concepts like division, multiplication, word grouping, or other cognitive strategies.  This paves the way for the fourth stage, in which children become capable of deduction and problem-solving. The important thing to note here is that although Piaget believed that these stages were natural, stemming from inborn, instinctive developmental "hardwiring" in the early stages, the speed and the success with which children progressed intellectually depended in large part on the extent to which these skills (like problem solving, grouping, and so on) were nurtured in their education.

What is Piaget's Cognitive Development theory?  

Jean Piaget's Cognitive Development theory states that human beings are born with impulses, instincts, and reflexes just like every other animal. In humans, however, these impulses and instinctive behaviors are the product of a higher intellectual capacity that all humans develop both biologically and socially.

According to Piaget, there is a scheme or pattern that all humans follow during their biological and psychological development. It consists of four steps, each having its own purpose and goal. Depending on how well-adjusted is the individual to his or her environment, the development will carry on successfully.

The steps are:

1. Sensorimotor stage (Infancy).  This is the stage with which we mostly associate the behavior of babies. This state alone is subdivided among 6 sub-stages. It is characterized by the discovery and exploration of the immediate environment, for the small connections that begin to be made, and for the use of memory, movement, and early language. Sensory means "feelings,emotions, senses", and motor stands for "mobility". Hence,the name of the stage.
2. Pre-operational stage (Toddler and Early Childhood). This stage refers to the growing period that pertains to children that are 2-4 years of age. It is characterized by the child's use of objects to build understanding through concrete representations that expand the imagination. During this time children also make conclusions based on what they immediately perceive (for example: All apples are red, or chocolate milk is already brown, etc). This is the stage of the challenging ages known as the "terrible two's and the "trying three's". Its complete, primal experimentation.
3. Concrete operational stage (Elementary and early adolescence). This is the school-age to High School stage in which most of the individual's intellectual development and social interaction occur. The operational thinking also develops, meaning that children of this age group are able to build, produce, and perform an action, and are also able to reverse it, if needed be.
4. Formal operational stage (Adolescence and adulthood). During this stage one experiences the responsibilities of high school, college, and young adulthood. It is a time where students can operate in the abstract, and do not need that many concrete representations to understand a concept. This last stage continues to develop throughout life.

Piaget's theory is the backbone of most educational theories today. It is accepted that students will go through a series of developmental and cognitive changes that will necessitate for a standardized way to develop instruction based on those needs.

Although society is changing as a whole, and the field of psychology consistently finds new information about the human brain, Piaget has  consistently served as the accepted foundation on which new strategies of academia are created, and tested.