Parkinson’s disease (PD) is a progressive neurodegenerative disorder characterized by loss of dopaminergic neurons and the pathological accumulation of misfolded alpha-synuclein (α-syn) within Lewy bodies and Lewy neurites, Retrospective neuropathologic and biochemical studies have shown that α-syn is not a monolithic structure, but can be assembled as a heterogeneous, structurally distinct fibrillar conformations referred to “strains” with unique biochemical properties, seeding activities, cellular tropisms, and neurotoxic potentials, and strain heterogeneity is gaining increased recognition as contributing to the heterogeneity of PD. Evidence from postmortem brain studies, amplification studies (i.e., PMCA, RT-QuIC) on diseased brain tissue, and biofluid studies also support the notion that α-syn strains lead to distinct clinical presentations, which includes PD, dementia with Lewy bodies (DLB) and multiple system atrophy (MSA). Retrospective findings suggest that strain switching may be a phenomenon occurring during the progression of the disease and associated with cognitive decline. Understanding strain diversity not only adds to our understanding of α-syn misfolding and PD pathogenesis, but it also provides an opportunity to develop strain-specific biomarkers and therapeutic targets. The purpose of the current review is to summarize the retrospective evidence for α-syn strain heterogeneity and discuss its potential important implications for diagnosis, prognosis, and precision medicine in PD.