Instructors are constantly baffled by the fact that two students who are sitting in the same class, who have access to the same materials, can come to understand a particular chemistry concept differently. Variation theory offers a theoretical framework from which to explore possible variations in experience and the resulting differences in learning and understanding. According to variation theory, there are a limited number of features of a given phenomenon to which we can pay attention at any given time. Our experience of that phenomenon depends on the specific features to which we direct our attention. Two individuals who experience the same phenomenon may focus on different features and, thus, come to understand the phenomenon differently. The purpose of this article is to present variation theory as (1) a useful way for instructors to think about student learning and (2) a potentially powerful theoretical framework from which to conduct chemical education research. (Contains 4 figures.)