Until recently, depression was considered an acute, self-remitting illness of limited duration. However, it is increasingly recognized as a chronic, relapsing, or recurrent illness in most patients, imposing a substantial burden on the individual and society. Research has demonstrated an association between depression and impaired psychosocial functioning, reduced productivity and life satisfaction, excess health care utilization, increased reporting of medically unexplained medical symptoms, comorbidity with other psychiatric illnesses, and suicide. It is more prevalent in patients with common chronic medical conditions, such as obstructive pulmonary disease, hypertension, diabetes, stroke, coronary artery disease, asthma, and chronic pain. In addition to its association with medical morbidity and mortality, depression has a negative impact on self-care behaviors, including medication adherence, in patients with chronic medical illnesses, and in health-related behaviors such as exercise, smoking, and weight control. As a consequence, it is a major barrier to effective care of chronic medical illnesses.