The first two sections of this chapter describe the greenhouse effect and its relationship to the evolution of the Earth and its atmosphere. Central to the issue of global warming is the Earth's energy balance and the molecular mechanism by which carbon dioxide and other compounds absorb the infrared radiation emitted by the planet. Some knowledge of molecular structure and shape is necessary to understand this mechanism. Therefore, the next sections develop a general method for predicting molecular geometry and relating that geometry to infrared-induced vibrations. The two sections that follow make it clear that most of the CO₂ in the atmosphere is of natural origin, but increased human contributions are the chief cause of the current concern about the enhanced greenhouse effect. These concerns have a significant quantitative component; we need numbers to help assess the seriousness of the situation. That need justifies several sections in which we introduce and illustrate some fundamental chemical concepts, — including atomic and molecular mass, Avogadro's number, and the mole concept. Examples and exercises demonstrate how the important ideas of mass and moles are related. Thus armed, we return to another look at several greenhouse gases. A discussion of predictions based on computer modeling of the climate leads to an assessment of the current situation. The chapter ends with some suggested answers to the all-important questions about global warming: "What can we do? What should we do?" Recognizing that global warming has international implications, should we and can we, as nations, collectively address global warming through initiatives such as the Kyoto Conference protocols? Finally, how does the issue of climate change compare with the issue of stratospheric ozone depletion considered in the previous chapter?