Lewy body diseases include Parkinson disease and dementia with Lewy bodies and are characterized by the widespread distribution of Lewy bodies in virtually every brain area. The main component of Lewy bodies is alpha-synuclein (AS). Accumulating evidence suggests that AS oligomerization and aggregation are strongly associated with the pathogenesis of Lewy body diseases. AS is a small soluble protein with aggregation-prone properties under certain conditions. These properties are enhanced by posttranslational modifications such as phosphorylation, ubiquitination, nitration, and truncation. Accordingly, Lewy bodies contain abundant phosphorylated, nitrated, and monoubiquitinated AS. However, alternative splicing of the AS gene is also known to modify AS aggregation propensities. Splicing gives rise to four related forms of the protein, the main transcript and those that lack exon 4, exon 6, or both. Since AS structure and properties have been extensively studied, it is possible to predict the consequences of the splicing out of the two aforesaid exons. The present review discusses the latest insights on the mechanisms of AS posttranslational modifications and intends to depict their role in the pathogenesis of Lewy body diseases. The implications of deregulated alternative splicing are examined as well, and a hypothesis for the development of the pure form of dementia with Lewy bodies is proposed.