The potential energy surfaces for the thermal reactions of bicyclo[3.2.0]hept-2-ene and norbornene have been explored with density functional theory at the Becke3LYP/6-31G* level. Both concerted and diradical pathways for the retro-Diels-Alder reaction of norbornene have been examined, and the activation parameters and C-13 primary kinetic isotope:effects predicted for the concerted pathway are in excellent agreement with experimental data. The concerted mechanism is favored over the lowest energy stepwise diradical route by 12.4 kcal/mol. For,the orbital symmetry-allowed suprafacial-inversion (si) pathway of the [1,3] sigmatropic rearrangement;of bicyclo[3.2.0]hept-2-ene to form norbornene, a mechanism involving a transition state which leads to a broad diradical plateau on the potential energy surface is predicted. Implications of these surfaces, which differ substantially from those obtained by semiempirical calculations, are also discussed.