Photochromic compounds are currently recognized as the promising candidates for future optoelectronic devices, such as memories and switches. Photochemical pericyclic reactions between 1,3,5-hexatriene and 1,3-cyclohexadiene have been used in several ways to construct versatile photochromic compounds. Recent development of the 6 pi-electrocyclic photochromic systems is overviewed.