Photochromism is characterized as a reversible colour change of a chemical species upon photoirradiation. Although vast numbers of photochromic molecules have been so far synthesized, molecules which exhibit thermally irreversible photochromic reactivity are limited within a few families of compounds. Among the thermally irreversible photochromic molecules, diarylethene derivatives are the most promising candidates for applications to opto-electronic devices because of their fatigue resistance, high sensitivity and rapid response. In this review, characteristic photochromic performances of the derivatives, detection and analysis of the photochromic reactions at a single-molecule level and application of the single-crystalline photochromism to molecular machinery will be described.