The aggregation of proteins into amyloid fibrils is associated with several neurodegenerative diseases. In Parkinson's disease it is believed that the aggregation of α-synuclein (α-syn) from monomers by intermediates into amyloid fibrils is the toxic disease-causative mechanism. Here, we studied the structure of α-syn in its amyloid state by using various biophysical approaches. Quenched hydrogen/deuterium exchange NMR spectroscopy identified five β-strands within the fibril core comprising residues 35–96 and solid-state NMR data from amyloid fibrils comprising the fibril core residues 30–110 confirmed the presence of β-sheet secondary structure. The data suggest that β1-strand interacts with β2, β2 with β3, β3 with β4, and β4 with β5. High-resolution cryoelectron microscopy revealed the protofilament boundaries of ≈2 × 3.5 nm. Based on the combination of these data and published structural studies, a fold of α-syn in the fibrils is proposed and discussed.