The rotational barriers for biphenyl, 2,2'-bipyridine and 2-phenylpyridine were calculated with HF, B3LYP, BPW91, and MP2 and double-zeta polarized basis sets. The methods differ significantly in the near-planar region, and are in close agreement otherwise, with DFT results similar to experiment over the whole range (for biphenyl). Steric repulsions, stabilizing C-H-N' interactions, pi-conjugation and electron correlation give rise to DFT-calculated dihedrals for biphenyl of about 40 degrees and similar 0 degrees and 90 degrees barriers, dihedrals of 23 degrees and an almost non-existent 0 degrees barrier for 2-phenylpyridine and two minima for 2,2'-bipyridine, which is strongly stabilized by two C-H-N' interactions in the coplanar conformation with anti-standing nitrogens.