With 1999 marking the 100th anniversary of the discovery of organic photochromic compounds1, it is a fitting time to reflect back on commercial uses that have developed from this interesting class of organic compounds over the years. From the many applications that have been proposed, including security devices and optical memory storage, the one application that has achieved substantial commercial success is switchable lenses. Such lenses, based on inorganic photochromes (AgX), have been available since the mid 1960s, but it is only in the last 10 years that organic systems with acceptable performance have been available in the marketplace. From the mid 1980s until his sudden death in early 1998, John Crano lead the research effort at PPG Industries that developed the photochromic systems that have been commercialized by Transitions Optical Inc. under the name Transitions®.