This chapter examines fiduciary principles in health care law. There is no unanimous agreement when it comes to the precise doctrinal consequences of labeling health care actors as fiduciaries in various contexts, such as personal injury, decisional authority, financial influence, and procedural rules. In the case of patients and physicians, certain attributes are said to constitute an archetypal fiduciary relationship, including agency, dependency, trust, and information asymmetry. Thus, many legal decisions and commentators argue that physicians have fiduciary responsibilities to patients. For courts, however, hospitals are not fiduciaries. They regard private hospitals as ordinary commercial enterprises. This chapter first provides an overview of arguments over whether physicians and non-physicians (for example, hospitals and health insurers) are “fiduciaries” before discussing health care fiduciaries’ duty of loyalty and duty of care, along with their other obligations such as duties of confidentiality and full disclosure. It also explores the ability of health care fiduciaries and patients to waive fiduciary duties, as well as how courts have addressed distinct causes of action for physicians’ breach of fiduciary duty. It shows that courts often invoke fiduciary concepts and terminology in discussing physicians’ obligations to patients, and that physician-patient (and other medical treatment) relationships have classic attributes of fiduciary status.