Diarylethenes undergo thermally irreversible and fatigue resistant photochromic reactions. In this lecture photochromic performance of the diarylethenes under two extreme conditions will be presented. One is the photochromism in the single-crystalline phase, and the other is at the single-molecule level. The diarylethene single crystals undergo very efficient photochromic reactions and exhibit physical property changes, such as surface morphology changes. One of the dreams of organic photochemists is to directly follow photochemical reactions at the single-molecule level. This was demonstrated using a diarylethene derivative linked with a fluorescent unit. The digital fluorescence switching of the single molecules is the first step to achieve the ultimate ultra-high density "Single-Molecule Optical Memory".