　　　A new family of thermally irreversible photochromic molecules, named “diarylethene,” was serendipitously discovered a quarter of a century ago. The diarylethenes undergo thermally irreversible, fatigue resistant, ultrafast and highly sensitive photochromic reactions. Many of diarylethenes can undergo photochromic reactions even in the single-crystalline phase. The outstanding photochromic performance attracts considerable attention from both scientific and technological points of view, and stimulates research from fundamental physico-chemical aspects to various applications. Here, it is reported how such thermally irreversible photochromic diarylethenes were discovered and developed.