The abnormal aggregation of proteins into fibrillar lesions is a neuropathological hallmark of several sporadic and hereditary neurodegenerative diseases. For example, Lewy bodies (LBs) are intracytoplasmic filamentous inclusions that accumulate primarily in subcortical neurons of patients with Parkinson's disease (PD), or predominantly in neocortical neurons in a subtype of Alzheimer's disease (AD) known as the LB variant of AD (LBVAD) and in dementia with LBs (DLB). Aggregated neurofilament subunits and alpha-synuclein are major protein components of LBs, and these inclusions may contribute mechanistically to the degeneration of neurons in PD, DLB and LBVAD. Here we review recent studies of the protein building blocks of LBs, as well as the role LBs play in the onset and progression of PD, DLB and LBVAD. Increased understanding of the protein composition and pathological significance of LBs may provide insight into mechanisms of neuron dysfunction and death in other neurodegenerative disorders characterized by brain lesions containing massive deposits of proteinacious fibrils.