Background: Fungal rhinosinusitis is a major clinical problem which should be considered in all patients and immunocompromised patients with chronic rhinosinusitis. It may be non-invasive or invasive with five main subtypes. Acute invasive fungal rhinosinusitis affects immunocompromised patients, usually with poorly controlled diabetes. Orbital and intracranial invasions are common, and mortality is high, except in cases of early detection and aggressive treatment. Chronic invasive fungal rhinosinusitis and chronic granulomatous fungal rhinosinusitis are characterized by a prolonged clinical course with slow progression of the disease, frequent invasion of the orbit and skull. Allergic fungal rhinosinusitis is a disease of young atopic individuals. There are usually involved all the sinuses with mucosa thinning and specific secretions. Fungus ball appears in one sinus, most frequently in the maxillary sinus, and affected individuals are not usually atopic. Anatomical and physiological interactions of the nose and paranasal sinuses form a dynamic system. Mucus is the first line of defense against inhaled irritants and pathogens. The hygiene of a normal airway is maintained by the mucociliary clearance. The immune system includes nasal epithelial surface properties, or non-specific innate immunity and specific acquired immunity. Conclusions: The detailed knowledge of anatomical, histological and immunological properties of the nasal and sinus mucosa is essential for understanding the pathophysiology of sinus diseases, treatment planning and surgical approach in order to obtain a favorable result.