The importance of preserving trust in physicians and in medical institutions has received widespread attention in recent years. Primarily, this is due to the threats to trust posed by managed care, but there is a general and growing recognition that trust deserves more attention than it traditionally has received in all aspects of medical ethics, law, and public policy. Trust has both intrinsic and instrumental value. Trust is intrinsically important because it is a core characteristic that affects the emotional and interpersonal aspects of the physician/patient relationship. As an instrumental value, trust is widely believed to be essential for effective therapeutic encounters. It has been hypothesized or shown to affect a host of important behaviors and attitudes relating to care, including seeking care, disclosing private information, complying with treatment, and being satisfied with care. Moreover, trust may be a mediator of measurable clinical outcomes and a key factor in the mind–body interactions that underlie placebo/nocebo effects and the effectiveness of alternative medicine. It is no surprise, then, that preserving, justifying, and enhancing trust is the fundamental goal of much of medical ethics, and is a prominent objective in healthcare law and public policy. Discussions of trust and related concepts were commonplace in professionally based medical ethics prior to the 1970s.This research was supported in part by the Robert Wood Johnson Foundation, under its Patient-Provider Relationship Initiative. Several colleagues contributed to the development of these ideas, including Elizabeth Dugan, Ph.D., Douglas Levine, Ph.D., Beiyao Zheng, Ph.D., Raj Balkrishnan, Ph.D., and Charles Rawlings, M.D. This article is based in part on a prior article and book chapter.Hall MA. The ethics and empirics of trust. In: Englehardt TH, Bondeson W, eds. The Ethics of Managed Care: Professional Integrity and Patient Rights. Amsterdam: Kluwer;2003:109–26.Hall MA. Law, medicine and trust. Stanford Law Review 2002;55:463–527.