The role of fungi in chronic rhinosinusitis (CRS) is not clear. Fungi can be detected in the nose and paranasal sinuses of virtually all CRS patients; however, they also appear to be present in healthy controls. Various theories attempt to explain the mechanisms by which fungi can exert an effect on sinus mucosa in susceptible individuals. Further studies are necessary to clarify the role of fungi in CRS, which fungal organisms (if any) are pathogenic, and what exactly characterizes the immunologic response to fungi that may result in the development of disease. However, in the absence of convincing immunologic data and evidence of clinical improvement of CRS after antifungal therapy, the case against the fungus remains unproven.