When considering a diagnosis of depression, a doctor looks for nine classic symptoms. A major depressive episode is present if five or more of the following nine symptoms are present during the same two-week period. At least one of the five symptoms must be either a depressed mood or loss of interest or pleasure.
1. Depressed mood for most of the day
2. Disturbed appetite or change in weight
3. Disturbed sleep
4. Psychomotor retardation or agitation
5. Loss of interest in previously pleasurable activities; inability to enjoy usual hobbies or activities
6. Fatigue or loss of energy
7. Feelings of worthlessness; excessive and/or inappropriate guilt
8. Difficulty concentrating or thinking clearly
9. Morbid or suicidal thoughts or actions

Mood Alterations
An alteration in mood is the "red flag" that alerts the doctor to consider a diagnosis of depression. Depressed individuals typically feel sadness or despair. They find they have lost their ability to feel pleasure and they are no longer interested in things they normally enjoy. Some depressed patients may be more irritable or tense than sad.

Emotional Changes
Many patients experience a marked lack of confidence and feelings of ineptitude. Some people will avoid situations that require any sort of responsibility for fear of failure.

Changes in the Way of Body Functions
Depression affects the body as well as the emotions. Depressed individuals may notice changes in the way their body functions. Typically, appetite is decreased; although, depression may be expressed as excessive appetite and weight gain. When people suffer from depression, they may have difficulty falling asleep, they may wake up in the middle of the night, or they may wake up in the early morning hours without being able to return to sleep. People who awake in the early hours (terminal insomnia) tend to have the most severe depression. Occasionally, depressed individuals complain of chronic fatigue and report excessive sleeping rather than insomnia.

Loss of Energy
Loss of energy and tiring too easily are also common symptoms of depression. Sex drive may be decreased markedly in depressed people and may lead to impotence and the inability to have an orgasm.

Difficulty Concentrating
Many people who suffer from depression find that they have difficulty concentrating or thinking clearly. Depressed individuals may feel that they are unable to study or that their efficiency at work is decreased. In severe cases, depressed individuals are not even able to watch television.

Masked Depression
It is possible that people suffer from major depression, while their friends and loved ones do not realize it. Their depression may be masked because they are not forthcoming about their low mood. Instead, they may complain of various physical problems (e.g. indigestion, heartburn, muscle or joint pains, and chronic headaches). Further questioning may reveal that depression is responsible for their symptoms.

Psychotic Symptoms
In very severe cases of depression, patients may develop psychotic symptoms such as hallucinations and delusions. Often the psychotic symptoms have depressive themes such as an unshakable belief that one is totally worthless. Some severely depressed patients hear a voice telling them to kill themselves.