Mental illness and mental retardation are completely two different terms and with two different concepts in them and cannot be used interchangeably. Mental illness can be caused by number of causes like genetic causes, environmental causes and chemical imbalances in the brain. While mental retardation in the imbalance in normal development of the brain in an individual by which it can be categorized into mild Estimates of the prevalence of comorbidity of psychiatric disorders and mental retardation in community and clinical populations range from 14.3 to 67.3 percent. Mental illness can be described when behaviour of individuals is inappropriate, irrational, or unrealistic in most physical, or in mental illness the person’s behaviour is not normal. Mental disorder can be caused by physical, psychological or environmental factors, or a com- bination of all three. While mental illness is not level of intelligence, it is possible for a person to be both mentally ill and retarded.