This book, Organic Photochromic and Thermochromic Compounds, is the fourth major treatise on photochromism involving organic molecules and derived systems. The first such book was edited by G.H. Brown in the Weissberger series in 1971, the second was edited by H. Durr and H. Bouas-Laurent in the Elsevier series in 1990. A third book, edited by C.B. McArdle, should be added to the list, which focuses on the very important topic of the behavior of photochromic systems in polymer matrices. The current book is a result of the large increase in the number of publications and patents concerning photochromic compounds and their use in various applications (e.g. ophthalmic lenses, security printing, etc.) during the last 10 years. As a result of this increased interest, two successful International Symposia on Photochromism have been held: the first, ISOP93, in France on Embiez Island near Bandol (September 12--16, 1993) and the second, ISOP96, in the United States in Clearw...