All possible structures of the hydrazine dimer have been investigated in a systematic way for all combinations of the two hydrazine conformers. Three structures with two hydrogen bonds, three with three hydrogen bonds and no structure with only a single hydrogen bond have been found. Two structures with four hydrogen bonds are saddle points of the potential energy surface. In the first step, the geometries of all investigated structures were optimized at the MP2/aug-cc-pVDZ level of theory. In the second step, the intermolecular parameters of all stable structures were determined at the CCSD(T)/aug-cc-pVTZ level. The binding energies of the six stable conformations include the BSSE correction. The frequency shifts of the antisymmetric wagging mode and the NH-stretching modes were calculated with the MP2/6-31++(2df,2p) method. The barrier heights of three rearrangement reactions were determined at the CCSD(T)/aug-cc-pVTZ level.