Apathy is a loss of motivation compared to the previous level of functioning of the subject. It affects the subject's behavior, cognition and emotional state. It is one of the main behavioral manifestations of Parkinson's disease. Although it may be a symptom of depression, it often exists as an isolated syndrome in Parkinson's disease patients. Apathy is usually not related to the severity of the motor symptoms, but frequently associated with the severity of cognitive impairment. Apathy is also a possible complication of treatment by stimulation of the subthalamic nucleus. Screening and assessment of apathy require the use of specific tools, some of which are validated in Parkinson's disease. From a pathophysiological point of view, apathy results from a dysfunction of the limbic circuit connecting the ventral striatum to orbitofrontal and anterior cingulate cortex. The dopaminergic denervation in these regions seems to play a key role, but other mechanisms are probably involved. Further studies are warranted to progress in the therapeutic management of this invalidating syndrome.