Peridontal diseases are infections, and many forms of the disease are associated with specific pathogenic bacteria which colonize the subgingival area. At least two of these microorganisms, Porphyromonas gingivalis and Actinobacillus actinomycetemcomitans, also invade the periodontal tissue and are virulent organisms. Initiation and progression of periodontal infections are clearly modified by local and systemic conditions called risk factors. The local factors include pre-existing disease as evidenced by deep probing depths and plaque retention areas associated with defective restorations. Systemic risk factors recently have been identified by large epidemiologic studies using multifactorial statistical analyses to correct for confounding or associated co-risk factors. Risk factors which we know today as important include diabetes mellitus, especially in individuals in whom metabolic control is poor, and cigarette smoking. These two risk factors markedly affect the initiation and progression of periodontitis, and attempts to manage these factors are now an important component of prevention and treatment of adult periodontitis. Systemic conditions associated with reduced neutrophil numbers or function are also important risk factors in children, juveniles, and young adults. Diseases in which neutrophil dysfunction occurs include the lazy leukocyte syndrome associated with localized juvenile periodontitis, cyclic neutropenia, and congenital neutropenia. Recent studies also point to several potentially important periodontal risk indicators. These include stress and coping behaviors, and osteopenia associated with estrogen deficiency. There are also background determinants associated with periodontal disease including gender (with males having more disease), age (with more disease seen in the elderly), and hereditary factors. The study of risk in periodontal disease is a rapidly emerging field and much is yet to be learned. However, there are at least two significant risk factors-smoking and diabetes-which demand attention in current management of periodontal disease.