Social and economic effects of mood disorders include functional impairment, disability or lost work productivity, and increased use of health services. Evidence for these impacts includes cross-sectional studies, longitudinal studies, and true experiments (randomized trials of specific treatments or treatment programs). With respect to unipolar depression, strong evidence demonstrates that depression is associated with significant functional impairment and that effective treatment helps to restore function. Studies of the effect of depression on work disability and health care costs show strong cross-sectional associations (i.e., greater disability and higher costs among those with depression) and longitudinal associations (i.e., improvement in depression is associated with reduced disability and lower costs). All of these findings regarding unipolar depression seem as consistent in the subgroup of patients with comorbid chronic medical illness as in the total population with depressive disorders. Fewer data are available regarding social and economic burden of bipolar disorder, but available data show cross-sectional associations between mood symptoms and functional impairment, disability, and health care costs. Taken together, these data describe the substantial social and economic burden of mood disorders and the potential benefits of more effective treatment. We must recall, however, that economic benefits of treatment for mood disorders are secondary to the principal objective of relieving human suffering.