Although the prevalence of major depression is about 1-4% among adults aged 65 or older, about 10% of elderly people have clinically significant depression symptoms in primary care settings. Only half of them have received appropriate treatment. Furthermore, 65% of old people have two or more chronic conditions such as heart disease, cancer, and diabetes. The effects between the comorbidities and depression are reciprocal. The comorbidities are associated with an increased risk of depression and may also interfere with the treatment of depression, while the depression may exacerbate the existing conditions and lead to worse outcomes and lower quality of life. Furthermore, national policy is not conducive to depression treatment. Medicare only reimburses about 50% of the total cost for psychotherapies such as behavioral treatment. It was not until 2006 that Medicare started reimbursing prescription drugs under the Part D program but with a large out-of-pocket expense. On the other hand, the availability of the generic versions of many new antidepressant drugs may reduce the financial barriers to the use of psychiatric medications. Using 2002 to 2007 Medicare Current Beneficiary Survey (MCBS), a national multistage panel survey linked with Medicare claims data, this study will examine the disease burden, the disparities in treatment patterns, determinants of mental health services use, and barriers to the treatment for depression among elderly Medicare beneficiaries with depression. In particular, this study will examine the detailed drug use patterns to assess the appropriateness of treatment, the interactions between comorbidities and health services use patterns, and the impact of national health insurance policy on the treatment patterns. This study addresses key quality of care issues in the treatment of late life depression during the transition period of national health.