Sinusitis is a very common chronic illness with a substantial health care impact. This review focuses on factors contributing to sinusitis pathogenesis and chronicity, including anatomic factors, disturbances in mucociliary clearance, microbial pathogens, and inflammatory factors. A distinction is made between “infectious” and “noninfectious” types of inflammation in chronic sinusitis. The inflammatory characteristics of noninfectious inflammation are reviewed primarily in the context of chronic hyperplastic sinusitis with nasal polyposis. Key features of this type of inflammation include the presence of chronic inflammatory cells, large numbers of eosinophils, and IL-5–producing T lymphocytes. Allergic fungal sinusitis is discussed as a special type of chronic sinusitis. Published studies on the outcomes of medical management are reviewed. Finally, algorithms for medical management of chronic sinusitis and allergic fungal sinusitis are presented. (J Allergy Clin Immunol 2000;106:213-27.)