The biochemistry of amyloid proteins has been a fascinating and important area of research because of its contribution to our understanding of protein folding dynamics and assembly and of the pathogenetic mechanisms of human disease. One such disease is Alzheimer's disease (AD), the most common neurodegenerative disorder of aging. In AD, the amyloid -protein (A), which is expressed normally and ubiquitously throughout life as a 40-42 residue peptide, forms fibrils that deposit in the brain as "amyloid plaques." This pathologic deposition process led researchers to investigate fibril formation as a target for therapeutic intervention. In doing so, an increasing number of fibril precursors and non-fibrillar A assemblies have been identified, the majority of which are neurotoxic. These findings have altered prevailing fibril-centered views of the pathobiology of amyloid diseases and intensified efforts to understand the early folding and assembly dynamics of A. We seek here to introduce the reader to the complex world of A assembly and biological activity, a goal we hope will provide a conceptual framework upon which further knowledge or experimentation may be built.