In recent years, two developments have imparted a new direction to research on the aetiology and pathogenesis of Parkinson's disease. First, the discovery that a missense mutation in the α-synuclein gene is a rare genetic cause of Parkinson's disease. Second, the identification of the α-synuclein protein as the main component of Lewy bodies and Lewy neurites, the defining neuropathological characteristics of all cases of Parkinson's and several other diseases. The filamentous inclusions of multiple system atrophy are also made of α-synuclein. These findings have placed α-synuclein dysfunction at the centre of several common neurodegenerative diseases. Here, I review the molecular properties of the synucleins, the different diseases characterized by the accumulation of α-synuclein, and the possible mechanisms by which dysfunction of α-synuclein might lead to neurodegeneration.