Chromogenic materials change their optical properties in response to an external stimulus. Due to their potential applications as light valve (e.g., in sun protecting glazing) or as temperature sensor, thermotropic, and thermochromic materials have come more and more into the focus of research and development. Commercialization of such high-tech products has already started and certainly many others will enter the market in near future. This book is the first to give an overview of the scientific and applicative aspects of the entire class of thermochromic and thermotropic materials. It discusses the origin of the thermo-optical effects at the molecular level and presents the macroscopic optical and material properties of chromogenic materials as well as their present and potential future application. With a view to particular potential applications, the book outlines the specific development strategies of these materials. The book addresses scientific and application-oriented researchers as well as students in the fields of smart adaptive polymers and sun-protecting materials. By providing the fundamental knowledge and outlining the future trends of thermochromism, this book familiarizes the readers with the entire field of the phenomenon.