I consider the idea of a structure of fundamental physical particles (as described, for example, in quantum theory) being causal. Causation is traditionally thought of as involving relations between entities—objects or events—that cause and are affected. On structuralist interpretations, however, it is unclear whether or how precisely fundamental particles can be causally efficacious. On some interpretations, only relations (as opposed to entities) exist; on others, particles are ontologically dependent on their relations in ways that problematize the traditional picture. I argue that thinking about causal efficacy in this context generates an inevitable pattern of reasoning. To assess the cogency of a given structuralist proposal one must take a stand with respect to a significant metaphysical challenge. Two options then emerge: skepticism about the form of structuralism at issue; or a dissolution of the challenge by means of a contentious ontological primitive. I contend that the choice between these options cannot be forced on scientific or philosophical grounds alone.