Health sciences are in the midst of major transitions. Advances in sciences and technology over the last century have greatly increased the knowledge regarding the pathogenesis of periodontal diseases. Periodontal disease is an infectious disease, but environmental, physical, social and host stress may affect and modify disease expression. Certain systemic conditions clearly may affect the initiation and progression of gingivitis and periodontitis. The concept of periodontal diseases as a localized entities affecting only the teeth and supporting apparatus is oversimplified and in need of revision. Rather than being confined to the periodontium, periodontal diseases may have wide-ranging systemic effects. In susceptible individuals, however, periodontal infections may act as independent risk factor for systemic diseases and may be involved in basic pathogenic mechanism of these conditions. Furthermore, periodontal infection may exacerbate existing systemic diseases. Proper use of the knowledge of potential relationships between periodontal diseases and systemic diseases and may be involved in basic pathogenic mechanism of these conditions.