Learnability in Valiant’s PAC learning model has been shown to be strongly related to the existence of uniform laws of large numbers. These laws define a distribution-free convergence property of means to expectations uniformly over classes of random variables. Classes of real-valued functions enjoying such a property are also known as uniform Glivenko–Cantelli classes. In this paper, we prove, through a generalization of Sauer’s lemma that may be interesting in its own right, a new characterization of uniform Glivenko–Cantelli classes. Our characterization yields Dudley, Gine´, and Zinn’s previous characterization as a corollary. Furthermore, it is the first based on a simple combinatorial quantity generalizing the Vapnik–Chervonenkis dimension. We apply this result An earlier version of this paper appeared in Proceedings of the 34th Annual Symposium on the