Alzheimer's disease (AD) is a progressive, neurodegenerative disorder characterized by amyloid deposition in the cerebral neuropil and vasculature. These amyloid deposits comprise predominantly fragments and full‐length (40 or 42 residue) forms of the amyloid β‐protein (Aβ) organized into fibrillar assemblies. Compelling evidence indicates that factors that increase overall Aβ production or the ratio of longer to shorter forms, or which facilitate deposition or inhibit elimination of amyloid deposits, cause AD or are risk factors for the disease. In vitro studies have demonstrated that fibrillar Aβ has potent neurotoxic effects on cultured neurons. In vivo experiments in non‐human primates have demonstrated that Aβ fibrils directly cause pathologic changes, including tau hyperphosphorylation. In concert with histologic studies revealing a lack of tissue injury in areas of the neuropil in which non‐fibrillar deposits were found, these data suggested that fibril assembly was a prerequisite for Aβ‐mediated neurotoxicity in vivo. Recently, however, both in vitro and in vivo studies have revealed that soluble, oligomeric forms of Aβ also have potent neurotoxic activities, and in fact, may be the proximate effectors of the neuronal injury and death occurring in AD. A paradigm shift is thus emerging that necessitates the reevaluation of the relative importance of polymeric (fibrillar) vs. oligomeric assemblies in the pathobiology of AD. In addition to AD, an increasing number of neurodegenerative disorders, including Parkinson's disease, familial British dementia, familial amyloid polyneuropathy, amyotrophic lateral sclerosis, and prion diseases, are associated with abnormal protein assembly processes. The archetypal features of the assembly‐dependent neuropathogenetic effects of Aβ may thus be of relevance not only to AD but to these other disorders as well. © 2002 Wiley‐Liss, Inc.