The most common neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease, are characterized by the misfolding of a small number of proteins that assemble into ordered aggregates in affected brain cells. For many years, the events leading to aggregate formation were believed to be entirely cell-autonomous, with protein misfolding occurring independently in many cells. Recent research has now shown that cell non-autonomous mechanisms are also important for the pathogenesis of neurodegenerative diseases with intracellular filamentous inclusions. The intercellular transfer of inclusions made of tau, α-synuclein, huntingtin and superoxide dismutase 1 has been demonstrated, revealing the existence of mechanisms reminiscent of those by which prions spread through the nervous system.