Alpha-synuclein is a small intracellular protein naturally abundant in the brain at low-micromolar concentrations. Its fibrillar aggregates are the major constituents of intracellular inclusions, known as Lewy bodies, which are the pathological hallmarks of Parkinson disease and related neurodegenerative disorders. However, increasing evidence suggest that oligomers, rather than fibrils, are the most toxic and damaging to brain neurons. Single molecule FRET can be used to detect and characterise the low levels of heterogeneous oligomers formed during protein aggregation. In this presentation, I will discuss the recent results of in-vitro studies of alpha-synuclein oligomer formation at physiologically-relevant concentrations using single-molecule FRET spectroscopy and show how these experiments reveal the key microscopic reactions taking place during the aggregation of alpha-synuclein.