The great ice sheets in Antarctica and Greenland are vast deposits of frozen freshwater that contain enough to raise sea level by approximately 70 m if they were to completely melt. Because of the potentially catastrophic impact that ice sheets can have, it is important that we understand how ice sheets have responded to past climate changes and will respond to present climate changes, so that we can predict the effect global warming will have on sea level rise in the coming centuries. The purpose of this article is to introduce students to some of the basic concepts of glaciology, the physical variables that control the evolution of ice sheets, and how changes in these parameters may affect the long-term evolution of ice sheets. (Contains 3 figures.)