We report on the results of optical, X-ray and NMR studies of light-induced phenomena in smectic phases triggered by molecular phototransformations. UV irradiation causes trans-to-cis isomerization of the dopant molecules while irradiation in the visible part of the spectrum facilitates cis-to-trans relaxation. Cis-to-trans isomerization decreases the smectic layers' spacing; as the result, a smectic slab undergoes an undulation instability and/or formation of focal conic domains. These textures are nonuniform and scatter light; this effect can be used for optical data storage. The opposite trans-to-cis isomerization disorders the orientation of molecules and increases the thickness of smectic layers. Two beams of different wavelengths with distinct yields of isomerisation can be used to write and erase optical information.