The structures, conformational energies, and vibrational frequencies of hydrazines have been studied at 6-31G** MP2 and 6-31G** B3LYP levels. Our theoretical structures generally agree with the available experimental data. However, our theoretical N-N bond length for tetramethylhydrazine is 1.436 Angstrom at the 6-31G** MP2 level, indicating that the experimental N-N distance (1.401 Angstrom) is too short. We confirm that the ab initio N-N bond length strongly depends on the torsional angle even at the correlated levels. The N-N bond lengths are much longer in the syn and anti conformations of 1,1-dimethylhydrazine than that in the gauche conformation. MM3 parameters for hydrazines have been optimized to reproduce experimental or quantum mechanical structures, energies, and vibrational frequencies. The bond lengthening resulting from lone-pair repulsion was accounted for with a torsion-stretch interaction. The heats of formation for hydrazines were calculated via both MP2 and MM3 methods. The optimized MM3 force field was applied to several cyclic hydrazines, and good agreement with experiments was noted. We found that diequatorial dimethylhexahydropyridazine is 0.9 kcal/mol more stable than the axial, equatorial conformer, in agreement with the experimental value of 1.2 kcal/mol.