Within bioethics, two prevailing approaches structure how we think about the role of medical surrogates and the decisions that they must make on behalf of incompetent patients. One approach views the surrogate primarily as the patient's agent, obediently enacting the patient's predetermined will. The second approach views the surrogate as the patient's custodian, judging for herself how to best safeguard the patient's interests. This paper argues that both of these approaches idealize away some of the ethically relevant features of advance (...) care planning that make patient preferences so inscrutable and surrogate decision-making so burdensome. It proposes a new approach to surrogate decision-making, the Fiduciary Agency Approach. On this novel approach, the surrogate has authority to not only act on the patient's behalf as the patient's agent but also to decide on the patient's behalf as the patient's fiduciary. One upshot of this new approach is that surrogates must sometimes go against the expressed dictates of the patients' advance directives not necessarily because doing so would be in the patient's best interest but rather because doing so would best represent the patients' will. (shrink)