The Institute of Medicine’s report on Dying in America called for honoring treatment preferences near the end of life for seriously ill patients. To achieve this objective, the report recommended that patients, their family members, other loved ones, and providers engage in shared decision making about current and future treatment decisions (that is, advance care planning). Yet decision science research suggests that preferences are objectively difficult to specify for complex contingencies and subjectively difficult to specify for unfamiliar choices. Because advance care planning involves both difficulties—the future may unfold in complex ways and pose unprecedented choices—it may not fully and faithfully specify patients’ preferences. I discuss a powerful but overlooked influence on this planning: local providers’ practice norms. Norms often begin as generally accepted procedures but evolve into rules enforced by both external and internal sanctions (such as shame and pride). Local practice norms regarding the timing, content, and interpretation of advance care planning conversations influence patient choice. While the influence of providers on patients’ decisions cannot be entirely removed, I recommend increasing providers’ awareness of this influence by using audit, feedback, and coaching and by systematizing processes for advance care planning.