The defining characteristic of a bubble is unsustainable growth that eventually reverses. Bubbles typically arise when uncertainty leads to unsustainable trends, and the authors argue that there are two areas in which higher education has experienced what appear to be unsustainable trends, namely, college costs (the costs to students, parents, and taxpayers of attending college, or alternatively, the spending of colleges) and enrollment. It is interesting that it is not clear which one really is unsustainable. If college costs are not a bubble, then it is likely that there is an enrollment bubble. On the other hand, if there is a bubble in college costs, then it is quite likely that there is not an enrollment bubble. In this article, the authors first present the argument for each separately then discuss the interaction between the two. (Contains 1 figure and 19 footnotes.)