At present, when a clinical diagnosis of Parkinson's disease (PD) is made, serious damage has already been done to nerve cells of the substantia nigra pars compacta. The diagnosis of PD in its earlier stages, before this irreversible damage, would be of enormous benefit for future treatment strategies designed to slow or halt the progression of this disease that possibly prevents accumulation of toxic aggregates. As a molecular biomarker for the detection of PD in its earlier stages, alpha-synuclein (α-syn), which is a key component of Lewy bodies, in which it is found in an aggregated and fibrillar form, has attracted considerable attention. Here, α-syn is reviewed in details.