Trust is one of the central features of patient-physician relationships. Rapid changes in the health care system are feared by many to be threatening patients' trust in their physicians. Yet, despite its acknowledged importance and potential fragility, rigorous efforts to conceptualize and measure patient trust have been relatively few. This article presents a synopsis of theories about patient trust and the evolution of methods to measure it. Clinicians, educators, and researchers interested in this area may find this information useful in practice and teaching. The gaps identified in our knowledge about trust can help target new efforts to strengthen the methodological basis of work to understand this vital element of medical relationships.