Even brilliant colors are all bound to scatter, Who in our changing world can stay forever? From Iroha-uta, ancient Buddhistic poem of Japan For many years we have been engaged in the preparation and characterization of new metal complexes and chelates, and especially the interpretation of their electronic spectra in solutions. In the course of these studies, we have encountered a number of strange changes in color which occur upon heating, cooling or compressing the solutions, or changing the nature of the solvent. Similar effects of temperature and pressure on the color were often also observed in the solid state. Records of visual observations, spectral measurements, and their interpretations and analyses accumulated each year, until we found ourselves, quite suddenly, in the middle of a fantastic world of color changes - the world of inorganic thermochromism and related chromo tropic phenomena. This book is a result of the reviews by Sone and Prof. S. Utsuno (Kagaku no Ryoiki, 22, 222 (1968); Bunko Kenkyu, 25,123 (1976)), and a series of papers by Fukuda, Sone et al. published in the 1. Inorg. Nucl. Chern., Bull. Chern. Soc. Japan, and various other journals after 1970.