　　　A new class of thermally irreversible photochromic molecules, named “diarylethene”, was serendipitously discovered a quarter of a century ago. The dithienylperfluorocyclopentene derivatives undergo thermally irreversible, fatigue resistant, ultrafast and highly sensitive photochromic reactions. The outstanding photochromic performance attracts considerable attention from both scientific and technological points of view, and stimulates researches on the diarylethenes from fundamental physico-chemical aspects to various applications in optical memories, switches and molecular machines. In this review development of studies on diarylethene molecules and crystals over the past 10 years is comprehensively described. More than 900 papers are cited and the total pages are 104.