This chapter in a forthcoming book justifies the conventional characterization of common-law agency as a fiduciary relationship. An agent serves as the principal’s representative in dealings with third parties and facts about the world, situating the agent as an extension of the principal for legally-salient purposes. A principal’s power to furnish instructions to the agent is the fundamental mechanism through which the principal exercises control over the agent, a requisite for an agency relationship. The agent’s fiduciary duty to the principal provides a benchmark for the agent’s interpretation of those instructions. The chapter draws on philosophical literature on the identity of the self over time to address related characteristics of a...