Four dinuclear rhenium complexes, [Re2Cl8](2-) (1), [Re-2(mu-Cl)(3)Cl-6](2-) (2a), [Re-2(mu-Cl)(3)Cl-6](-) (2b), and [Re-2(mu-Cl)(2)Cl-8](2-) (3), were theoretically investigated by the CASSCF, MRMP2, SA-CASSCF, and MCQDPT methods. Interesting differences in electronic structure and Re-Re bonding nature among these complexes are clearly reported here, as follows: In 1, the ground state is the (1)A(1g) state. The approximate stabilization energies by the sigma, pi, and delta bonding interactions are evaluated to be 4.36, 2.89, and 0.52 eV, respectively, by the MRMP2 method. In 2a, the ground state is the E-2 state. The approximate stabilization energy by two degenerate delta bonding interactions is estimated to be 0.36 eV by the MCQDPT method. One delta bonding interaction of 2a is much weaker than that of 1, which is discussed in terms of the Re-Re distance and the Re oxidation state. In 2b, the ground state is the (1)A(1) state, of which multiconfigurational nature is extremely large unlike that of the 2E ground state of 2a despite similarities between 2a and 2b. In 3, the sigma, pi, and delta bonding interactions are not effectively formed between two Re centers. As a result, the (1)A(g), B-3(1u), (5)A(g), and B-7(1u) states are in almost the same energy within 0.03 eV. This result is consistent with the paramagnetism of 3 experimentally reported.