The chemical composition of any planetary atmosphere is of fundamental importance in determining its photochemistry and dynamics in addition to its thermal balance, climate, origin and evolution. Divided into two parts, this book begins with a set of introductory chapters, starting with a concise review of the Solar System and fundamental atmospheric physics. Chapters then describe the basic principles and methods of spectroscopy, the main tool for studying the chemical composition of planetary atmospheres, and of photochemical modeling and its use in the theoretical interpretation of observational data on chemical composition. The second part of the book provides a detailed review of the carbon dioxide atmospheres and ionospheres of Mars and Venus, and the nitrogen-methane atmospheres of Titan, Triton and Pluto. Written by an expert author, this comprehensive text will make a valuable reference for graduate students, researchers and professional scientists specializing in planetary atmospheres.