More than twenty different human proteins can fold abnormally resulting in the formation of pathological deposits and several feared degenerative diseases. These proteins lack primary sequence homology, yet they can self-assemble into fibrils with a characteristic cross beta-sheet structure. Among them, beta-amyloid of Alzheimer's disease is the best characterized. beta-amyloid is the principal protein component of senile plaques seen in the brains of individuals diagnosed with Alzheimer's disease and is implicated in the neurotoxicity associated with the disease. Although Alzheimer's disease has been the center of intense research, much remains to be learned concerning the role of beta-amyloid and the toxicity mechanisms that mediate its biological responses. In the current review, the general background information on Alzheimer's disease and amyloid are first presented. Next, attention is focused on the relevant research works regarding structural aspect and molecular features of A beta species as well as beta-amyloid firibrillogenesis/aggregation process. Importantly, an emerging issue that an oligomeric intermediate is the primary toxic beta-amyloid species is also discussed. Lastly, several beta-amyloid-induced toxicity mechanisms are proposed and reviewed. It is our belief that the advances in basic understanding of the conformational changes as well as biological functions of beta-amyloid and beta-amyloid-elicited toxicity mechanisms will shed light on the development and/or design of potential interfering agents against amyloid formation associated with Alzheimer's disease.