The criteria for diagnosis for depression specify that a person is in a depressed mood and/or experience loss of interest or pleasure for two weeks or more. Five or more of the following symptoms may be experienced by the person most of the day almost everyday, such as depressed mood (irritable, feels sad or empty, appears tearful), markedly decreased interest or pleasure in activities he previously enjoyed, significant weight loss or weight gain, insomnia or hypersomnia, psychomotor agitation or retardation, fatigue or loss of energy, feelings of worthlessness or excessive and inappropriate guilt (which may be delusional), diminished ability to think, concentrate and decide, and recurrent thoughts of death, suicidal ideation without a specific plan or a suicide attempt. The symptoms can cause significant distress or impairment in social and occupational functioning. It is important to note that the symptoms are not caused by effects of any substance taken by the patient. Moreover, the symptoms cannot be accounted for by recent events such as bereavement after losing a loved-one, calamity, or losing a job, house, and the like, (Depression is normal after these events but should last less than two weeks).

The key symptoms of depression are a depressed mood and loss of interest and/or pleasure. The depression one feels is worse than the usual feelings of anguish and grief. The depressed person feels hopeless and worthless. He feels anxious. Others have difficulty crying or have many unprovoked crying spells. Others may just be plain passive and indifferent. Almost all depressed patients have reduced energy and they may find tasks at home, work or school too burdensome. They have difficulty finishing their work and may find no motivation to start anything new. A depressed patient may also complain of changes in his sleep pattern (e.g. trouble in sleeping or waking up in the morning or multiple awakenings at night, during which he contemplates on his situation). The depressed person may also complain of aches and pains that have no real physical causes. A majority of depressed patients have tried to kill themselves or thought about it.