The book is devoted to spectroscopy and photophysics of selected organic dyes in turbid fluid and opaque solid matrices. The first few chapters provide a short description of the instrumentation and experimental methods including the basics of the diffuse reflectance techniques, the photochemistry of thioketones in solution and on solid matrices, and the photochemistry of alloxazines and isoalloxazines in various environments. Specific practical applications of the results are presented in the two chapters that follow, including the spectroscopy and photochemistry of different softwood kraft pulps, and pyrene fluorescence quenching by halothane in poly (oxyethylene) -poly (oxypropylene) poly (oxyethylene) triblock copolymers. Apart from the original scientific results obtained by the author, this book offers a useful introduction to spectroscopy in non-transmissive matrices. Steady-state and time-resolved spectroscopic techniques adjusted to non-transparent samples are described, with a special emphasis on the diffuse reflectance laser flash photolysis. The outlooks and benefits of photochemical studies in solids are discussed, along with a few comments and guidelines. The results presented demonstrate a wide range of potential applications of optical spectroscopy in turbid and opaque media.