2) Piaget's developmental theory 

Piaget is a Swiss psychologist. He started his work just before second world war. The main research areas of Piaget were how children develop and learn. He compared his theory with previous laboratory experiments and he developed the theory from child’s observable behaviours. 

Piaget's Key Ideas

1. Adaptation What it says: adapting to the world through assimilation and accommodation
2. Assimilation The process by which a person takes material into their mind from the environment, which may mean changing the evidence of their senses to make it fit.
3. Accommodation The difference made to one's mind or concepts by the process of assimilation.  
4. Note that assimilation and accommodation go together: you can't have one without the other. 
5. Classification The ability to group objects together on the basis of common features. 
6. Class Inclusion The understanding, more advanced than simple classification, that some classes or sets of objects are also sub-sets of a larger class. (E.g. there is a class of objects called dogs. There is also a class called animals. But all dogs are also animals, so the class of animals includes that of dogs) 
7. Conservation The realisation that objects or sets of objects stay the same even when they are changed about or made to look different. 
8. Decentration
9. The ability to move away from one system of classification to another one as appropriate.
10. Egocentrism The belief that you are the centre of the universe and everything revolves around you: the corresponding inability to see the world as someone else does and adapt to it. Not moral "selfishness", just an early stage of psychological development. 
11. Operation The process of working something out in your head. Young children (in the sensorimotor and pre-operational stages) have to act, and try things out in the real world, to work things out (like count on fingers): older children and adults can do more in their heads. 
12. Schema (or scheme) The representation in the mind of a set of perceptions, ideas, and/or actions, which go together. 
13. Stage A period in a child's development in which he or she is capable of understanding some things but not others 

The Key Ideas of Piaget's

Piaget’s stages of Development

Stage	Stage defined	Explanation/ Implications
1 Sensorimotor	0-2 years	Use of imitation, memory and thought Recognize that objects do not cease to exist when they are hidden Moves from reflex actions to goal-directed activity
2 Preoperational	2-7 years	Develops use of language and ability to think in symbolic form. Think operations through logically one direction. Difficulties in seeing other persons viewpoint
3 Concrete Operational	7-12 years	Able to solve concrete (hands-on) problems in logical fashion, Understands the law of conversation and is able to classify and seriate. Understands reversibility
4 Formal Operrational	12 plus years	Able to solve abstract problems in logical fashion. Becomes more scientific in thinking. Develops concerns about social issues, identity. Overcoming talent develops Own beliefs and attitudes develops gradually

Therefore the students should be treated according to his / her stages of the development. Because the Cognitive development results from the interactions that children have with their physical and social environments.

Piaget's Developmental Theory: An overview (Davidson Films - A video)

Video of overview of Piaget's Developmental Theory

Piaget's approach is central to the school of cognitive theory known as "cognitive constructivism": other scholars, known as "social constructivists", such as Vygotsky and Bruner, have laid more emphasis on the part played by language and other people in enabling children to learn.

Criticisms of Piaget’s Stage Theor

1) Stages of learning are too rigid ( the stage implications may differ from child to child and it can not be generalized

2) Individual differences ignored. (The effect of cultural and SES (Socia-Economic Status) on Learning is ignored)

3) Piaget gave little importance on construction of new knowledge through social interaction and constructivist ideas.

Conclusion

Piaget's Developmental Theory is based on the type of learning behaviour by the children according to stage of development. In other words it is about the achievement of milestones by the children through out their developmental stages of life.

Based on the findings in this theory, my opinion for the teachers is to develop activities and other learning approaches according to the developmental stage of the child. It should not be too high or even too low.