Major depression and depressive symptoms, although commonly encountered in patients with medical illnesses, are frequently underdiagnosed and undertreated in women with breast cancer. Depression and its associated symptoms diminish quality of life, adversely affect compliance with medical therapies, and reduce survival. Treatment of depression in women with breast cancer improves their dysphoria and other depressive symptoms, enhances quality of life, and may increase longevity. In this review, studies that investigate pathophysiologic alterations in patients with cancer and comorbid depression are discussed, and the few studies on treatment of depression and related symptoms in women with breast cancer are examined.