Alpha-synuclein is an abundant neuronal protein that has been linked both to normal synaptic function and to neurodegeneration. Most significantly, mutations in the gene encoding for alpha-synuclein are responsible for Parkinson's disease (PD) in rare familial cases, and the aggregated protein is a major component of Lewy bodies found in sporadic PD. Here we review recent data regarding the structure, the regulation at the transcriptional and posttranslational level, and the physiologic and aberrant functions of alpha-synuclein. We focus in particular on the fibrilization potential of alpha-synuclein and on its link with defects in protein degradation.