We have studied the diner of amyloid beta peptide A beta of 40 residues by means of all-atom replica exchange molecular dynamics. The A beta-dimers have been found to be the smallest toxic Species in Alzheimer's disease but :their inherent flexibilites have precluded structural characterization by experimental methods. Though the 24-mu s-scale simulation reveals a mean secondary structure of 18% beta-strand and 10% alpha helix we find transient configurations with an unstructured N-terminus and multiple beta-hairpins spanning residues 17-21 and 30-36 but the antiparallel and perpendicular peptide orientations are preferred over the parallel organization. Short-lived conformational states also consist of all alpha topologies and one compact peptide with beta-sheet structure stabilized by a rather extended peptide with alpha-helical content. Overall this first all atom study provides insights into the equilibrium structure of the A beta 1-40 dimer in aqueous solution, opening a new avenue for a comprehensive understanding of the impact of pathogenic and protective mutations in early stage Alzheimer's disease on a molecular level.