This chapter discusses the greenhouse effect and climate evolution using Venus as an example. Venus mostly has an Earth-like inventory of volatile gases—the basic ingredients of atmospheres and oceans—but with one glaring exception: water. Earth's ocean is 300 times as massive as its atmosphere. Water is more abundant than all the other volatiles combined, including carbon dioxide, nitrogen, and oxygen. In contrast, Venus has only a small amount of water, and it is all in the atmosphere. The chapter first compares Earth and Venus in terms of size, distance from the Sun, and bulk composition, as well as climates and inventories of hydrogen, oxygen, carbon, nitrogen, and sulfur. It then considers loss of water and escape of atmospheres on Venus, along with the so-called runaway greenhouse.