Parkinson's disease is the second most common neurodegenerative disease, and results from loss of dopaminergic neurons in the substantia nigra. The aggregation and fibrillation of alpha-synuclein in the form of intracellular proteinaceous aggregates (Lewy bodies and Lewy neurites) have been implicated as a causative factor in this disease, as well as in several other neurodegenerative disorders, including dementia with Lewy bodies, Lewy body variant of Alzheimer's disease, multiple system atrophy and Hallervorden-Spatz disease. Thus, the aggregated forms of alpha-synuclein play a crucial role in the pathogenesis of the synucleinopathies. However, the molecular mechanisms underlying alpha-synuclein aggregation into specific filamentous inclusions remained unknown until recently. Data on the aggregation and fibrillation properties of human alpha-, beta- and gamma-synucleins, mouse alpha-synuclein and familial Parkinson's disease mutants of human alpha-synuclein (A30P and A53T) are analyzed in order to shed light on the amino acid determinants of synuclein aggregation.