A survey is given of various factors and conditions that affect the photoreactivity and photoproduct distribution of conjugated trienes, such as ground-state rotameric equilibrium, non-planarity of the chromophore, wavelength of irradiation, solvent and temperature. The effect of changes in ground-state conformation and geometry, caused by substitution or variation of solvent or temperature, usually appears to be more important and more easily predictable than excited state effects.