The four giant planets of the solar system fall into two classes: the gaseous giants, Jupiter and Saturn, mostly composed of hydrogen and helium, and the icy giants, Uranus and neptune, which are less massive and mostly made of ices. This chapter reviews their thermal structures, their chemical compositions, and their cloud structures. The abundance ratios of the giant planets, inferred from their spectral properties, are reviewed, and the implications of these parameters in terms of formation and evolution models are discussed.
