The ring-opening of cyclobutene (a) has been studied by methods based on density functional theory (DFT). Both the simple local spin density approximation (LDA) and a more extensive nonlocal (NL) approach has been applied. A detailed analysis of the atomic movements in the ring-opening process was further obtained by intrinsic reaction coordinate (IRC) calculations. The IRC study indicates that the ring-opening is a concerted process in which the rotation of the CH2 groups and the breaking of the C-C sigma-bond take place simultaneously. Attention has also been given to the number of conformers for the 1,3-butadiene ring-opening product. The species s-trans-1,3-butadiene (e) of C-2h symmetry was calculated to be the most stable conformer. The only other conformer calculated to be stable was a nonplanar cis-1,3-butadiene species (c) of C-2 symmetry. Planar s-cis-1,3 butadiene was found to be a transition state connecting the interconversion of c to its enantiomer. Structures, relative energies and frequencies are provided for the species a-e as well as the ring-opening transition state b. The DFT estimates were further compared with results from ab initio calculations on the species a-e. The ab initio calculations were based on up to fourth-order Moller-Plesset perturbation theory (MP4) and large basis sets of 6-311G** and 6-311+G** quality. It was found that the DFT methods provide as accurate estimates as the high level nb initio calculations of the properties for a-e. The DFT frequencies compare better with the experimental values than the MP2 figures. The LDA method affords a good estimate of the barrier height for the forward reaction a --> c. However it underestimates the heat of reaction by as much as 5 kcal/mol. The NL corrections remove this error, and for the overall relative energies, the NL results are comparable to the MP4 estimates.