The fact that psychological factors and psychiatric disorders may affect the clinical course of medical illness is incontrovertible and is no longer the topic of serious debate. For example, psychiatric disorders may negatively affect outcome, increasing adverse events, length of stay in general hospital patients, and health care costs (Daumit et al. 2006; Walker et al. 2003). A striking example is the increased morbidity and mortality associated with major depression in patients with coronary artery disease (Frasure-Smith et al. 1993). Although most of the consultation-liaison literature has focused on interrelations between comorbid psychiatric and medical disorders, a wealth of epidemiological research has identified behavioral risk factors for the development of medical illness. Behavioral factors such as cigarette smoking, obesity, alcohol and substance dependence, and hazardous sexual practices are major causes of premature death and medical morbidity both in the United States and worldwide. A description of areas of investigation for classifying the psychological, behavioral, and social factors that may affect physical health is presented in Table 24–1.