The various protein deposits of brain amyloidosis share common ultrastructural, biophysical, and histological properties. These amyloidogenic deposits can be composed of distinct proteins, which are conceptually associated with different neurodegenerative diseases. Amyloidogenic proteins are typically soluble monomeric precursors, which undergo remarkable conformation changes associated with the polymerization into 8- to 10-nm wide fibrils, which culminate in the formation of amyloid aggregates. Some amyloidogenic inclusions are extracellular, such as senile plaques of Alzheimer's disease, which are composed of amyloid beta (Abeta) peptides. However, intracytoplasmic amyloid aggregates, such as neurofibrillary tangles in Alzheimer's disease and Lewy bodies in Parkinson's disease, are composed of the proteins tau and alpha-synuclein, respectively. The mounting awareness that the latter proteins are directly linked to the etiology of spectrum of neurodegenerative diseases has resulted in the coining of the terms "tauopathies" and "synucleinopathies." However, emerging evidence for the overlap in the pathological and clinical features of patients with brain amyloidosis suggests that they may be linked mechanistically. Recently, it was demonstrated that alpha-synuclein, which has the ability to readily form amyloid in vitro without the need of other co-factors, can initiate tau amyloid formation. Following this initiation step, alpha-synuclein and tau can synergize the polymerization of each other. Furthermore, increased levels of Abeta peptides in brain can promote the formation of intracellular tau and alpha-synuclein amyloid aggregates, although the mechanism for this process is still unclear. These results indicate that the formation of amyloid composed of different proteins can affect each other directly or indirectly, likely contributing to the overlap in clinical and pathological features.