The atmosphere is the gaseous outer portion of a planet. Atmospheres have been detected around all planets and several satellites, and each is unique. Some atmospheres are very dense, and gradually blend into fluid envelopes which contain most of the planet's mass. Others are extremely tenuous, so tenuous that even the best vacuum on Earth seems dense in comparison. The composition of planetary atmospheres varies from the solar-like hydrogen/helium envelopes of the giant planets to atmospheres dominated by nitrogen, carbon dioxide, or esoteric gases such as sulfur dioxide or sodium for terrestrial planets and satellites of giant planets. However, even though all atmospheres are intrinsically different, they are governed by the same physical and chemical processes. For example, clouds form in many atmospheres, but with vastly different compositions since the gases available to condense differ. The upper layers of an atmosphere are modified by photochemistry, with the particulars depending on atmospheric composition. Variations in temperature and pressure lead to winds, which can be steady or turbulent, strong or weak. The various processes operating in planetary atmospheres are discussed in this chapter, and the characteristics of the atmospheres of bodies within our Solar System are summarized.