Ab initio and rate theory calculations are used to study the OH + HO2 → O2 + H2O reaction. The structure and energy are determined for the planar HO⋯HO2 hydrogen-bonded complex. At the highest level of theory considered here, PMP4/6-31G**, this complex is more stable than the separated reactants by 5.1 and 9.0 kcal/mol on the 3A″ and 3A′ potential energy surfaces, respectively. The 3A″ surface correlates with the ground electronic state products, and the energy and vibrational frequencies for the reaction transition state on this surface are calculated at the HF/6-31G** and MP2/6-31G** levels. In nonplanar configurations an avoided crossing occurs between the 3A′ and 3A′ surfaces. The nonplanar 3A transition state which arises from the 3A″ transition state is characterized at both the HF and MP2 levels. However, there is considerable uncertainty regarding the position of the 3A′-3A″ avoided crossing. As a result, it is not clear whether the triplet reaction transition state originates from the planar 3A″ transition state or the 3A′-3A″ avoided crossing. The reaction kinetics on the triplet surface is controlled by the long-range attractive potential and not by the reaction transition state. A vibrationally/rotationally adiabatic model, with proper treatment of electronic degeneracies, gives a reaction rate constant as a function of temperature which agrees within a factor of 2 with the most recent experimental results for the 298-1100 K temperature range. Combining this calculated temperature dependence with the average of all recent experimental measurements of the OH + HO2 reaction rate at ambient temperature yields the rate constant k1 = 7.1 × 10-11(T/300)0.21 exp(113/RT) cm3 molecule-1 s-1. This expression produces substantially larger rate constants at high temperature than those previously estimated and, we believe, represents a new preferred rate constant for use in combustion modeling.