Radiation can be absorbed and re-emitted many times in atomic vapors before it reaches the boundaries of the container encasing the vapor. This effect is known as radiation trapping. It plays an important role practically everywhere atomic vapors occur, whether in spectroscopy, gas lasers, atomic line filters, the determination of atomic lifetimes, measurements of atomic interaction potentials, or electric discharge lamps. This book is the first to assemble all of the information necessary to handle practical problems related to radiation trapping, and it emphasizes both physical insights and mathematical methods. The introduction reviews resonance radiation and collision processes in atomic vapors. This is followed by detailed explanations of the physical effects and mathematical methods for various types of problems (e.g., with or without saturation, particle diffusion, reflecting cell walls). The last part of the book describes the applications of these methods to a variety of practical problems, such as cross-section measurements and the design of discharge lamps.