Ab initio valence bond calculations are performed for the allyl cation, radical, and anion with 6-31G* basis set. Delocalized and hypothetically localized structures of these systems are thoroughly optimized and analyzed. The delocalization energies, defined as the energy difference between the delocalized structure and its hypothetically localized one, for the three allyl systems are -55.7, -28.4, and -52.3 kcal/mol, respectively. Our results clarify the recent debate on whether the allyl anion has little or comparable resonance stabilization with the allyl cation. The methylene rotation barriers of the allyl cation, radical, and anion are successfully explained in terms of resonance, hyperconjugation, and rehybridization. For the allyl radical, its resonance energy is only about half those of the allyl cation and anion; thus, it has the lowest rotation barrier. The twisted allyl cation, in which the rotating methylene group is perpendicular to the C-C-C plane, has the highest hyperconjugation energy (-6.8 kcal/mol), while, in its twisted form, the allyl anion has a negligible hyperconjugation effect. As the allyl anion assumes its twisted form, the carbon atom in the rotating methylene experiences a remarkable rehybridization from sp(2) mode in the planar form to sp(3) mode. This process decreases the total energy of the twisted allyl anion as much as 14.3 kcal/mol and eventually makes its rotation barrier smaller.