The results of periodontal researches in the recent 10 years have made us to come to a better understanding of the dental plaque associated periodontal diseases (including gingivitis and periodontitis). Gingivitis can appear in any ages. General speaking, the prevalence and severity of gingivitis increase with age and the eruption of permanent dentition. Periodontitis can also occur in children, during prepuberty and puberty. Since the early diagnosis and execution of prevention of a disease is the most effective program of control the disease, it is very important to understand the nature of periodontal diseases in children and adolescents.