The potential energy hypersurface (PES) of the reaction OH + OH (+M) -> H2O2 (+M) has been investigated at the CASPT2/aug-cc-pVDZ and CASPT2/aug-cc-pVTZ levels of theory. The PES is characterized by a barrier below the energy of the reactants and a hydrogen-bonded adduct formed by the OH radicals. On the basis of the potential energy hypersurface obtained, the high-pressure limiting rate coefficient (L) of the reaction was calculated using variable reaction coordinate transition-state theory, classical trajectory simulations, and a two-transition-state model. Over the temperature range of 200-3000 K, k(infinity)(T) = 9.3 x 10(-9)T(-1.040) exp(3.5/T) + 1.13 x 10(-12) T-0.313 exp(84/T) cm(3) molecule(-1) s(-1) is reported. Available experimental data on the pressure dependence of the reaction with He and Ar as bath gases were analyzed using a two-dimensional master equation. Over the temperature range of 200-3000 K, the following low-pressure limiting rate coefficient (k(0)) and center broadening factor (F-cent) were obtained for He as the bath gas: k(0)(T) = 4.4 x 10(-20)T(-4.30) exp(-340/T) cm(6) molecule(-2) s(-1) and F-cent = 0.54. For the dissociation of H2O2 in Ar, the following values are reported over the temperature range of 500-3000 K: k(0)(T) = 1.4 x 10(8)T(-4.57) exp(-26322/T) cm(3) molecule(-1) s(-1) and F-cent = 0.55. The calculations describe all experimental data well, except the observations at 210 K for the reaction with He as the bath gas.