Astronomy is an observational science, not an experimental one. As a result, theory plays a particularly important role in the field. It provides the basic paradigms within which observations are framed and without which they degenerate into catalogs of uninterpreted data. At its most satisfying, theory makes predictions that are later verified by observation. But theory can also have a dramatic “postdictive” impact by explaining previously observed phenomena. It can catalyze specific new observations, which in turn stimulate models. Finally, theory provides much of the conceptual stimulation that invests astronomy with excitement.