The desire to understand photochemical processes in heterogeneous environments has led to a considerable scientific interest in studies of photophysics and photochemistry of organic molecules adsorbed on different substrates both by steady-state and time-resolved methods. Among the time-resolved techniques, traditional laser flash photolysis has been increasingly applied in various transparent media; however, it fails in opaque systems. Diffuse reflectance laser flash photolysis extends the advantages of laser flash photolysis to non-transmissive media and enables transient spectra and kinetics of photoinduced elementary reactions to be determined in heterogeneous environments. Since its invention twenty years ago, the diffuse reflectance laser flash photolysis has remained one of the most important tools to study opaque media. To begin with, the article offers a useful introduction to spectroscopy of opaque media. It provides a concise description of the instrumentation and experimental methods applicable to opaque systems, including the basics of the diffuse reflectance techniques. 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. Next, the article presents selected results covering different possible applications, obtained mostly by the present authors. These results include spectroscopy and photophysics of selected organic dyes on opaque solid supports, such as photochemistry of thioketones on solid matrices, spectroscopy of some solid metal-ligand complexes, and photochemistry of iso- and alloxazines in various opaque environments. Results on complex systems of practical importance are presented, including spectroscopy and photochemistry of different softwood kraft pulps, and pyrene fluorescence quenching by halothane in poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) triblock copolymers. The examples given show a wide range of potential applications of optical spectroscopic techniques in turbid and opaque media, based on the diffuse-reflectance approach.