This article reviews the history of allergic fungal rhinosinusitis and the clinical, pathologic, and radiographic criteria necessary to establish its diagnosis and differentiate this disease from other types of chronic rhinosinusitis. Allergic fungal rhinosinusitis is a noninvasive fungal form of sinus inflammation characterized by an often times unilateral, expansile process in which the typical allergic "peanut-butter-like" mucin contributes to the formation of nasal polyps, hyposmia/anosmia, and structural changes of the face. IgE sensitization to fungi is a necessary, but not sufficient, pathophysiologic component of the disease process that is also defined by microscopic visualization of mucin-containing fungus and characteristic radiological imaging. This article expounds on these details and others including the key clinical and scientific distinctions of this diagnosis, the pathophysiologic mechanisms beyond IgE-mediated hypersensitivity that must be at play, and areas of current and future research.