Severe depression, also known by the names clinical depression, unipolar depression and major depression, is a mental disorder. The American Psychiatric Association has given it the term “major depressive disorder”. Low mood, loss of interest in normally enjoyable activities, low self-esteem characterizes severe depression. People suffering from depression remain continually sad. A depressed person often feels unworthy, helpless and hopeless about their ability to change the state of affairs. Severe depression is a type of depression; the term depression as such is inclusive of other types of psychological depression also. People suffering from severe depression are disabled to such an extent that their professional and personal life, eating and sleeping habits, and general health and lifestyle are adversely affected. The figures are alarming. In the United States, 60% of people who commit suicide are severely depressed, and 3.4% of people who are in a state of severe depression commit suicide. The subject of depression, its nature and causes is ever-evolving. The understanding is incomplete and several causes have been identified. These include psycho-social, psychological, biological, hereditary and evolutionary causes. This is also known as the bio-psychosocial model and it states that different factors cause depression to different extents, depending on the situation.