There is evidence from numerous studies, using both psychophysical and electrophysiological approaches, that impairment of olfaction is a characteristic and early feature of Parkinson's disease (PD). Deficits in the sense of smell may precede clinical motor symptoms by years and can be used to assess risk for developing PD in otherwise asymptomatic individuals. Recent data indicate that over 95% of patients with PD present with significant olfactory loss. Thus, olfactory dysfunction should be considered a reliable marker of the disease. This review summarizes the available information about olfactory function in PD, indicating the advantageous use of olfactory probes in early and differential diagnosis.