The photochemical cyclization of butadiene to cyclobutene is used to illustrate certain problems connected with the application of the Woodward-Hoffmann rules to photoinduced concerted reactions. Energy levels for the ground states and excited states of butadiene, cyclobutene, and a large number of intermediate configurations have been calculated, thus enabling the establishment of potential surfaces, from which probable reaction paths for the thermal and photoinduced reactions can be determined. It appears that the thermal disrotatory process is improbable since this would require an activation energy of ca. 100 kcal/mole. Further, it turns out that the driving force for the photoinduced ring closure as a disrotatory process derives from a favorable symmetric excited state, whereas Woodward and Hoffmann consider an antisymmetric excited state.