A major hallmark of Alzheimer's disease (AD) is the aggregation of amyloid-beta peptides in the brains of people afflicted by the disease. The exact pathway to this catastrophic event is unknown. In this work, a total of 9.5 mu s molecular dynamics simulations have been performed to investigate the structure and dynamics of the smallest form of toxic A beta oligomers, i.e., the A beta dimers. This study suggests that specific hydrophobic regions are vital in the aggregation process. Different possible structures for A beta dimers are reported along with their relative binding affinity. These data may be used to design better A beta-aggregation inhibitors. The diversity of the dimer structures suggests several aggregation pathways.