Major depressive disorder is one of the most commonly seen psychiatric disorders all over the world. If it is not treated, major depressive disorder may lead to early death, worsening in general health and considerable disability. The etiology and physiopathology of major depressive disorder has not been able to be illuminated. Multiple factors are important in the etiology of depression. Also, it should be considered that there are a few clinically different sub-types of depression whose diagnosis can sometimes be difficult. Different peculiarities of depression accompanying diseases, especially in older population, which are different from the depression observed in other age groups, are remarkable. For this reason, not only psychiatrists but also other clinicians should be aware towards diagnosis and treatment of depression. Because, as the depression, accompanying general medical diseases, which is not diagnosed and treated properly negative effects the course and prognosis of those diseases, extends the length of hospitalization. What you wish to know about major depressive disorder which causes disability and economic losses in adult population takes place in this book.