Alpha-synuclein (alpha-syn) appears to normally regulate neurotransmitter release, possibly via calcium-dependent binding and dissociation from lipid domains on secretory vesicles. The pathogenic effects of alpha-syn leading to Parkinson's disease (PD) appear to result from alternate toxic effects on lipid membrane. A variety of findings indicate that overexpression of wild-type alpha-syn, pathogenic mutations of alpha-syn, and dopamine-modified-alpha-syn promote toxic interaction between alpha-syn oligomers and lipids. These may disrupt transmembrane concentration gradients across secretory vesicles and other organelles and interfere with normal lysosomal or ubiqutin/proteasome mediated protein degradation or mitochondrial function. Additional causes of PD may interfere at other points with normal handling and degradation of alpha-syn, providing a variety of entry points to a converging neurodegenerative path underlying the disease.