Lipophlic and water-soluble dendrimers have been synthesized and their photochemical and photophysical characteristics have been studied. For example, the triplet energy migration was able to be achieved even at very low concentration by introducing the sensitizer intramolecularly into the dendrimer structure, and the reactiion process was clarified. Furthermore, water-soluble stilbene and diphenylbutadiene dendrimers have also been prepared and their photochemical reaction have been studied. Thus, highly efficient trans-to-cis photochemical isomerization reactions have been observed.