The number of patients with mood disorders including depression has increased by 700,000 over the past decade. But, the rate of consultations with psychiatrists is still low. It is thought that depression patients manifest not only psychiatric symptoms but also various other symptoms such as insomnia, fatigue, anorexia, and headache. On the other hand, chronic diseases such as cancer, chronic pain, cardiac disease, diabetes, dementia, and epilepsy tend to cause depression, and side effects of drugs such as interferon (IFN) and steroids can also cause depression. Early detection and treatment are effective in depression. It is important that general physicians detect depression in patients at an early stage and provide early intervention in collaboration (liaison) with psychiatrists. However, the diagnosis of depression is mainly by a psychiatrist interview but not by objective criterion such as blood tests. Thus, the depression biomarkers to determine the onset and severity are needed. In this symposium, we will introduce the development of depression biomarkers based on the findings from basic research using animal models of depression, and their application to clinical research in the field of psychiatric liaison.