Optical Spectroscopy is an interdisciplinary science dedicated to generating and building knowledge in the field of spectroscopy and as a tool for practical investigations. This book has been elaborated for the use of specialists, students and young scientists interested in material characterization and direct investigation of various technological processes. As the title shows, the project of this book is ambitious and challenging. The topics have been selected to supply the physical background needed to understand the main spectroscopic principles and, at the same time, to reveal new potentiality for applications. Subjects like Reflectance Spectroscopy, Infrared Attenuated Total Reflection or Photoreflectance Spectroscopy provide an overview of classical methods in spectroscopy in contexts of new applications and reveal possibilities in new domains such as medicine, environmental investigations, etc. Various spectroscopic measurement methods embedded in characterization of materials, devices or technological processes are extensively presented.