This invention relates to inhibitory peptides which bind to α-synuclein molecules and inhibit α-synuclein amyloidogenic aggregation, α-synuclein cytotoxicity, and spread of α-synuclein. Methods of making and using the inhibitory peptides (e.g. to treat subjects having conditions or diseases that are mediated by α-synuclein, such as Parkinson's disease, dementia with Lewy bodies, or MSA) are described.