Potential energy diagrams for ground and excited states have been calculated approximately for trans stilbene and several geometrical configurations, including a planar cis form, generated by twisting about the central double bond. The calculations show that on twisting the lowest triplet $^3$B becomes the most stable state, and that the $\pi$-electron distribution of this state in the perpendicular model is equivalent to separation into two benzyl systems. The lowest singlet $^1$A that corresponds to the ground state in cis and trans stilbene does not separate in this way. Quantitative agreement is obtained with observed thermal and spectroscopic evidence. The potential energy diagrams provide a satisfactory interpretation of the qualitative features of the ultraviolet and fluorescent spectra, the absence of phosphorescence, and the mechanisms of the thermal, the photochemical and the photosensitized isomerization reactions.