Periodontal disease consists of a group of infections affecting the gingiva and the supporting structures: cementum, periodontal ligament and alveolar bone. Gingival and periodontal changes in children could be due to the plaque and certain systemic diseases, which have a direct effect on the periodontium. The gingiva and periodontium of children differ in some respects from those of adults. Hence, periodontal diseases in children can be studied from anatomical, microbiological and cellular aspects. This chapter covers the anatomic considerations in children, histopathology of gingivitis in children, microbiology of periodontal diseases in children, and classification of periodontal diseases in children. Periodontal diseases in children are classified as gingival lesions, types of periodontitis, and periodontitis associated with syndromes.