The physiology of olfaction remains less well understood than that of the other special senses. Odorant molecules traverse the nasal vault to reach the cribriform area and become soluble in the mucus overlying the exposed dendrites of receptor cells. Anatomic blockage of the nasal cavity with subsequent airflow disruption is the most common cause of olfactory dysfunction (hyposmia or anosmia). Polyps, septal deformities, and nasal tumors may be the cause. Transient olfactory dysfunction often accompanies the common cold, nasal allergies, and perennial rhinitis through changes in the nasal and olfactory epithelium. About 20% of olfactory dysfunction is idiopathic, although it often follows a viral illness. Central nervous system neoplasms, especially those that involve the olfactory groove or temporal lobe, may affect olfaction and must be considered in patients with no other explanation for their hyposmia or for other neurologic signs. Head trauma is a rare but severe cause of olfactory dysfunction. Shearing of the olfactory neurites accounts for less than 5% of cases of hyposmia but is more commonly associated with anosmia. Absent, diminished, or distorted smell or taste has been reported in a wide variety of endocrine, nutritional, and nervous disorders In particular, olfactory dysfunction in (eg, Parkinson disease and Alzheimer disease) has been the subject of research as neurofibrillary tangles and Lewy bodies are found throughout the olfactory system. Quantitation of olfactory dysfunction may serve as a useful marker of disease progression and response to specific treatment.