Concerns about trust are long-standing, although mistrust may be increasing. Daily life could not flourish without trust. It can be defined in a variety of ways, but health-care programs have not demonstrated proven interventions by which it can be increased. It is intrinsically and instrumentally valuable. Good health care requires trust in systems as well as individuals. Trustworthiness is a virtue, whereas not all are trusted even if trustworthy. Trust functions at different levels of knowledge. An ethos of mistrust leads to a contractual relationship, with an infinite regress as to where one places trust. There is a paradox between trust and rationality: this depends on how rationality is construed, but in some situations we may trust against the evidence. The uncertain outcomes in medicine mean that sometimes trust may lead to disappointment, but trust should not automatically be abandoned.