Many paradigms concerning the epidemiology, pathogenesis, and systemic impact of periodontal diseases have been modified. For example, bacterial biofilms are essential to induce periodontitis, but their mere presence is not sufficient to initiate disease. It is also now recognized that the host response to these biofilms causes most of the destruction of the periodontal tissues. Codeterminants that influence the clinical severity of the disease process include environmental, genetic, and acquired factors. In general, the prevalence of advanced periodontitis and the incidence of disease progression are lower than previously believed. However, periodontitis remains the most common chronic illness. In addition, the finding that acquired systemic diseases may predispose individuals to periodontitis, and conversely that periodontitis may be a risk factor for certain systemic diseases, has expanded the scope of periodontics. These changed paradigms and their consequences with regard to selecting therapies are discussed in this review article.