Parkinson's disease (PD), one of the most frequent neurodegenerative disorders, is a progressive multi-organ proteinopathy caused by misfolded α-synuclein (αSyn) with variegated motor and nonmotor deficits owing to a spreading process of synaptic and neuronal loss in the nervous system. The motor core deficits of PD including rigidity, akinesia, rest tremor, and postural instability are attributed to the loss of dopaminergic nigrostriatal system, while the nonmotor alterations, such as hyposmia, autonomic and other dysfunctions frequently antedating motor symptoms are linked to widespread distribution of αSyn in the central, autonomic and peripheral nervous system and multiple organs. Recent studies have shown that αSyn aggregation in presynaptic terminals that predates the formation of Lewy bodies (LB), the characteristic markers of PD, is a key event in the pathogenesis of PD and other synucleinopathies. Progress in our understanding of the underlying mechanisms include insights into the functional organization of the basal ganglia and related cortico-subcortical circuits and their relations with morphological and pathophysiological lesions in the nervous system. The pathomechanisms underlying the cardinal motor abnormalities and nonmotor manifestations are briefly reviewed.