Introduction Loss of smell is involved in various neurologic and neurodegenerative diseases, such as Parkinson disease and Alzheimer disease. However, the olfactory test is usually neglected by physicians at large. Objective The aim of this study was to review the current literature about the relationship between olfactory dysfunction and neurologic and neurodegenerative diseases. Data Synthesis Twenty-seven studies were selected for analysis, and the olfactory system, olfaction, and the association between the olfactory dysfunction and dementias were reviewed. Furthermore, is described an up to date in olfaction. Conclusion Otolaryngologist should remember the importance of olfaction evaluation in daily practice. Furthermore, neurologists and physicians in general should include olfactory tests in the screening of those at higher risk of dementia.