The aggregation of alpha-synuclein (αS) protein from soluble monomer into solid amyloid fibrils in the brain is associated with a range of devastating neurodegenerative disorders such as Parkinson’s disease. Soluble oligomers formed during the aggregation process are highly neurotoxic and are thought to play a key role in the onset and spreading of disease. Despite their importance, these species are difficult to study by conventional experimental approaches owing to their transient nature, heterogeneity, low abundance and a remarkable sensitivity of the oligomerisation process to the chosen experimental conditions. In this thesis, well-established single-molecule techniques have been utilised to study the aggregation and oligomerisation of αS in solution.