Mental retardation is delayed and finally incomplete mental development. So overall level of intelligence, i.e., cognitive, language, motor, and social abilities are impaired. Mental retardation is often a permanent condition, still some cases are reversible. Depending upon the severity it can be classified into mild, moderate, severe and profound types.

Autism, disintegrative disorders are included here. These show qualitative abnormalities in reciprocal social interactions and in patterns of communications. The outcome of most of the illnesses in this category is poor.

ADHD i.e. attention deficit hyperactivity disorder, disturbance of activity and attention. These children are hyperactive - always on the move, but their attention is poor, i.e. they are highly distractible. They disturb there whole surrounding, and are often a nightmare to have them in the house or the class.

This is a group which is often an eye sore for the society and gives the media a lot of opportunity to write about. These children show a repetitive and persistent pattern of dissocial, aggressive, or defiant behavior. Their behavior is more serious than ordinary childish mischief or adolescent rebelliousness. They often violate the law and many land up in juvenile reformation homes. Depending upon the situation in which the misdeeds are done, the conduct disorders are classified as those, confined to the family context (the abnormal behavior is almost entirely confined to interactions with members of the nuclear family or immediate household, unsocialized conduct disorder (done alone, i.e., without company), socialized conduct disorder (misdeeds are done in company of others, i.e., in a gang type of situation), and oppositional conduct disorder (although these children are markedly defiant, disobedient and provocative, they are unlikely to break the law or violate the rights of others).

A tic is an involuntary (not a deliberate act), rapid, recurrent, non-rhythmic motor movement (i.e., movement of some part of the body). Tics can be motor and/or vocal tics, this includes Tourette's disorder.