The potential energy surface of the benzene dimer was studied by ab initio methods with inclusion of correlation energy. Three minima of importance were localized on the surface and a further one at a higher energy. Surprisingly the most stable structure was found to be the parallel-displaced structure at all theoretical levels followed by two T-shaped structures, one normal and one displaced. The normal T-shaped structure is slightly less stable. The energy barriers among the three most stable minima are very low, so that all are in dynamic equilibrium. The experimental intermolecular distance (4.96 Angstrom) determined for the T-shaped structure agrees nicely with the respective theoretical value of 5.0 Angstrom. The theoretical stabilization enthalpy (2.3 kcal/mol) supports one of the experimental values based on bulk properties of benzene (2.4 kcal/mol). The stabilization enthalpy derived from experimental measurements of appearance potentials (1.6 +/- 0.2 kcal/mol) is slightly below the theoretical value here. The structural preponderance of the T-shaped and parallel-displaced forms even for larger systems-and their semiquantitative understanding as quadrupole-quadrupole interactions-also is seen to carry through to complex aromatic-aromatic interactions in solid benzene as well as in the crystalline proteins, showing these interactions to be of generic importance.