Self-assembly of amyloid-beta (A beta) peptides in human brain tissue leads to neurodegeneration in Alzheimers disease (AD). Amyloid fibrils, whose structures have been extensively characterized by solid state nuclear magnetic resonance (ssNMR) and other methods, are the thermodynamic end point of A beta self-assembly. Oligomeric and protofibrillar assemblies, whose structures are less well-understood, are also observed as intermediates in the assembly process in vitro and have been implicated as important neurotoxic species in AD. We report experiments in which the structural evolution of 40-residue A beta (A beta 40) is monitored by ssNMR measurements on frozen solutions prepared at four successive stages of the self-assembly process. Measurements on transient intermediates are enabled by ssNMR signal enhancements from dynamic nuclear polarization (DNP) at temperatures below 30 K. DNP-enhanced ssNMR data reveal a monotonic increase in conformational order from an initial state comprised primarily of monomers and small oligomers in solution at high pH, to larger oligomers near neutral pH, to metastable protofibrils, and finally to fibrils. Surprisingly, the predominant molecular conformation, indicated by C-13 NMR chemical shifts and by side chain contacts between F19 and L34 residues, is qualitatively similar at all stages. However, the in-register parallel beta-sheet supramolecular structure, indicated by intermolecular C-13 spin polarization transfers, does not develop before the fibril stage. This work represents the first application of DNP-enhanced ssNMR to the characterization of peptide or protein self-assembly intermediates.