Studies on photocotrol of various functions at molecular level are described. The photoresponsive systems should be composed of both photosensitive and functional elements. The photosensitive elements are usually molecules which show photochromism such as azobenzene and spiropyran. The functional groups mentioned here are enzymes, inhibitors, receptors, cyclodextrins and crownethers, and the controlled functions are catalytic activity, membrane potential, viscosity, pH, conductivity, complex formation, chiral recognition and binding of metal ions.