Family systems theory provides the physician with a comprehensive paradigm to investigate the social and interpersonal context of physical illness. All physical illnesses, particularly life-threatening and chronic ones, profoundly affect the family life of the patient, and the changed family circumstances in turn have impact upon the physically ill patient. This reciprocal interaction between the illness and the family system varies according to the onset, course, outcome, and level of disability. This paper discusses the general application of family systems theory to physical illness as well as the description of the family dimension of acute life-threatening illness, AIDS, chronic illness, chronic neurologic disease, dementia, chronic pain, and cancer in children. We propose that an operational knowledge of theoretical concepts and technical competencies of family systems theory can enhance the therapeutic reach of physicians and other health professionals.