Chronic rhinosinusitis (CRS), a disease presenting with chronic symptoms such as nasal obstruction, rhinorrhea, hyposmia and facial pain, is highly prevalent and has a considerable impact on quality of life and health care expenditures. The disease is characterized by chronic inflammation of the sinonasal mucosa and can present with nasal polyps. Current consensus classifies CRS into CRS with nasal polyps and CRS without nasal polyps. This review illustrates the diversity of pathophysiological observations in CRS and highlights selected etiological hypotheses. A wide spectrum of alterations is described regarding histopathology, pattern of T cells and inflammatory effector cells, remodeling, immunoglobulin production, chemokine and eicosanoid production, and the role of microorganisms. The pathophysiological diversity observed in CRS seems to stand in contrast to its nonspecific clinical presentation, but is of the utmost importance in the development and application of highly individualized treatments. Identification of specific disease subgroups and their etiologies is an important and challenging task for future research.