The detailed dynamical processes involved in trans --> cis photoisomerization of butadiene have been studied in realistic simulations, employing a technique that is described in the text. Many interesting features are observed, including the following sequence of events: (i) The initial electronic excitation converts the central single bond to a double bond and the terminal double bonds to single bonds, so the molecule at first rotates about only these end bonds. (ii) There is then a series of rapid nonadiabatic transfers of population among electronic states near the HOMO-LUMO gap, which ultimately result in depopulation of the excited states. (iii) The bonds consequently revert to their original ground-state character, permitting a continuous rotation about the central single bond. At the end, the molecule is essentially in the ground electronic state for the new conformation. The simulation results clearly demonstrate the couplings of C-C-C bending vibrations to nonadiabatic electronic transitions and involvement of hydrogen migration in the molecular orbital intersections.