Publisher Summary This chapter discusses that significant advances in our understanding of the photochromic properties of naphthopyrans that have been made in the past decade Such is the structural diversity of naphthopyrans now available that it is possible to achieve almost any shade of any color. Furthermore, control of the fade rate with half-lives of less than a second to many hours is possible, thereby supplementing the fast and strong coloration and increasing the potential applications of these long-lived materials. Their facile synthesis from readily available precursors has allowed the commercial development of naphthopyrans to proceed at a good pace through the efforts of a small number of research groups linked to international manufacturers. The major end user is the ophthalmic industry. Brown or grey sun lenses can now be based on a single photochromic naphthopyran, whereas less than 20 years ago, multicomponent lenses were the norm. Naphthopyrans are used in security inks and as identity markers in a variety of products. Future uses in cosmetic applications and as devices are under development.