According to what I call the reductive standard-causal theory of agency, the exercise of an agent's power to act can be reduced to the causal efficacy of agent-involving mental states and events. According to a non-reductive agent-causal theory, an agent's power to act is irreducible and primitive. Agent-causal theories have been dismissed on the ground that they presuppose a very contentious notion of causation, namely substance-causation. In this paper I will assume, with the proponents of the agent-causal approach, that substance-causation is possible, as I will argue against that theory on the ground that it fails as a theory of agency. I will argue that the non-reductive agent-causal theory fails to account for agency, because it fails to account for agential control: it cannot explain why the stipulated irreducible relation between the agent and an action constitutes the agent's exercise of control over the action. This objection, I will argue, applies to the agent-causal theory in particular, and to the non-reductive approach in general.