Overall, while most clinicians would agree that aggressive forms of periodontitis exist as clinical entities, the clinical distinction between chronic and aggressive periodontitis (especially generalized) is not clear cut. This may not be all that significant from a treatment perspective, in so far as individualized anti-infective therapies are effective for both forms of the disease. However, from a research perspective, it is essential that these diseases be clearly distinguished in order to gain a complete understanding of their etiology and pathogenesis. The relative lack of clinical inflammation often associated with the localized molar-and-incisor form of aggressive periodontitis has been commented on for almost 100 years, and it is generally accepted that this form of the disease is associated with a thin biofilm, at least in its early stages. In contrast, the presence of clinical inflammation in generalized aggressive periodontitis appears to be similar to that observed in chronic periodontitis, and in this situation age of onset and family history are important additional criteria for either diagnosis or classification. It is also generally recognized that chronic periodontitis may subsequently be superimposed on both localized and generalized forms of aggressive periodontitis. While this may have little bearing on the treatment of such cases, it could have an enormous impact on both the design and interpretation of research studies, whether basic science or clinical. This highlights the essential difference between a diagnosis and a classification, whereby a diagnosis is the clinician's best guess, leading on to a treatment plan, whereas a classification does not allow such flexibility, requiring non-overlapping case definitions for research purposes if the underlying etiology of these diseases is ever to be fully elucidated.