The hallmark event of Alzheimer's disease (AD) is the deposition of amyloid as insoluble fiber masses in extracellular neuritic plaques and around the walls of cerebral blood vessels. The main component of amyloid is a hydrophobic peptide, named amyloid beta-peptide (beta A4), which results from the processing of a much longer membrane amyloid precursor protein (APP). This review focuses on the structural features of beta A4 and the factors that determine beta A4 insolubilization. Theoretical and experimental studies of the primary structure of beta A4 have shown that it is composed of a completely hydrophobic C-terminal domain, which adopts beta-strand structure, and an N-terminal region, whose sequence permits different secondary structures. In fact, this region can exist as an alpha-helical or beta-strand conformation depending on the environmental condition (pH and hydrophobicity surrounding the molecule). The effects of pH and hydrophobicity on beta A4 structure may elucidate the mechanisms determining its aggregation and amyloid deposition in AD.