Functional medicine is essentially patient centered, rather than disease centered. A structure is presented for uniting a patient-centered approach to diagnosis and treatment with the fruits of modern clinical science (which evolved primarily to serve the prevailing model of disease-centered care). The core scientific concepts of disease pathogenesis are antecedents, triggers, and mediators. Antecedents are factors, genetic or acquired, that predispose to illness; triggers are factors that provoke the symptoms and signs of illness; and mediators are factors, biochemical or psychosocial, that contribute to pathological changes and dysfunctional responses. Understanding the antecedents, triggers, and mediators that underlie illness or dysfunction in each patient permits therapy to be targeted to the needs of the individual. The conventional diagnosis assigned to the patient may be of value in identifying plausible antecedents, triggers or mediators for each patient, but is not adequate by itself for the designing of patient-centered care. Applying the model of person-centered diagnosis to patients facilitates the recognition of disturbances that are common in people with chronic illness. Diet, nutrition, and exposure to environmental toxins play central roles in functional medicine because they may predispose to illness, provoke symptoms, and modulate the activity of biochemical mediators through a complex and diverse set of mechanisms. Explaining those mechanisms is a key objective of the Textbook of Functional Medicine (from which this article is excerpted). A patient's beliefs about health and illness are critically important for self-care and may influence both behavioral and physiological responses to illness. Perceived self-efficacy is an important mediator of health and healing. Enhancement of patients' self-efficacy through information, education, and the development of a collaborative relationship between patient and healer is a cardinal goal in all clinical encounters.