Multiconfigurational wave functions were used to study the (1) concerted conrotatory, (2) concerted disrotatory, and (3) nonconcerted isomerization processes of bicyclo[l. 1.0]butane (C4H6) to 1,3-butadiene. The barriers for (1), (2), and (3) are about 42, 56, and 116 kcal/mol, respectively, as calculated with the second-order multireference perturbation theory (PT2). The barriers obtained from the multireference CI (MRCI) are within 1 kcal/mol of the those predicted by PT2. The predicted conrotatory barrier is within 1 kcal/mol of the experimentally measured barrier. The predicted stereochemistry is in agreement with the experimental observations.