A Survey Is Given Ofvarious Factors and Conditions That Affect the Photoreactivity and Photoproductdistribution of Conjugated Trienes, Such As Ground-State Rotameric Equilibrium,Non-Planarity of the Chromophore, Wavelength of Irradiation, Solvent Andtemperature. the Effect of Changes In Ground-State Conformation and Geometry,Caused By Substitution or Variation of Solvent or Temperature, Usually Appearsto Be More Important and More Easily Predictable Than Excited State Effects.