From the distant, small, and icy bodies of the solar system, such as comets and Pluto, to the gas giants, Jupiter and Saturn, and the rocky inner planets (except Mercury), chemistry driven by the Sun plays a central role. Some of the most abundant constituents of the atmosphere become a principal source of a variety of minor constituents following the action of sunlight. The minor constituents, in turn, hold the key to the stability of the atmosphere, and they provide important clues to the planetary interiors, and transport, origin, and evolution of the planetary atmospheres. On Earth, trace species are central to an understanding of such questions as global climate change and the biosphere.