The structure and torsional barriers at 0 degrees and 90 degrees for biphenyl were studied by both ab initio and density functional methods by using various levels of theory with different basis sets. The torsional angle (phi) calculated at the MP2/6-311+G(2d,2p) level was 42.1 degrees, while phi calculated using various density functionals with different basis sets was close to 40 degrees. In contrast with the ab initio results, the torsional barrier at 0 degrees [DeltaE(0)=E(phi =0 degrees)-E(equilibrium)] obtained using various density functionals coincided well with experimental values. The torsional barrier ratio (DeltaE(90)/DeltaE(0)) obtained at the B3LYP/cc-pVTZ level, 1.0988, agreed well with the experimental value, 1.0833, whereas it was 0.416 at the MP2/6-311G(d,p) level. Structural studies on biphenyl ions showed that the biphenyl cation has a nonplanar (phi =19 degrees) structure whereas its anionic counterpart has a planar structure. The ionization potential obtained at the B3LYP/6-311+G(2d,2p) level was 7.86 eV. Contrary to an earlier study, a positive electron affinity (EA) was obtained, in accordance with experimental predictions. EA values of 0.021 and 0.076 eV were obtained at the B3LYP/6-311+G(2d,2p) and B3LYP/aug-cc-pVDZ levels, respectively.