The purpose of this paper is to compare the two most plausible interpretations of the meaning of "probability" as that term occurs within the context of statistical laws: the frequency interpretation (elaborated by Hans Reichenbach and Richard von Mises, for example) and the propensity interpretation (first proposed by Karl Popper and recently discussed by Ian Hacking, among others). Both interpretations assume there is an important connection between probabilities and frequencies, but they fundamentally differ in their conceptions of the nature of that connection. Once this basic difference is made explicit, it becomes apparent that choosing between them poses a dilemma resulting from a certain tension between desiderata of epistemological and of systematic character, respectively. My concern, therefore, is to contribute toward the resolution of this dilemma.