Major depressive disorder (MDD) - sometimes referred to as clinical depression - is one of the oldest diseases recognized in medicine. Since antiquity, physicians have recognized a condition of severely depressed mood that arises acutely, persists, and then lifts as quickly as it started. From reviewing the chronicles of this illness in literature, one finds that the course of the illness remains the same now as it has in the past. Present day studies have revealed several findings. First, the average duration of a depressive episode is six months, although an episode can last longer. Second, the average age of patients at the onset of the illness is in the fourth decade of life, though MDD can present at any age. Third, the illness affects about twenty percent of the population; that is, one-fifth of the population will suffer a depressive episode in their lives, and all ethnic groups are affected. Fourth, the illness tends to affect females more than males. Fifth, it is a chronic illness. There is an eighty percent chance that a person will have another episode of depression during his or her lifetime and the average number of episodes in such patients is four. Last, individuals with a family history of MDD are more apt to have depressive episodes than the general population.

MDD is an illness of that presents with psychological and physical symptoms. The psychological symptoms include persistent sadness, irritability or feelings of emptiness, loss of interest in pleasurable activities, a decreased ability to concentrate, feelings of worthlessness or helplessness, and, most troubling, recurrent thoughts of death and suicide: a particularly important point, since these later symptoms are not normally found in people suffering from bereavement or grief. (To note, the depression associated with bereavement or grief is always related to a loss or perceived loss, and the severity and length of depression is proportional to the precipitant. This is in contrast to MDD where there may not be a precipitant.) In severe cases of depression, a patient may have psychotic symptoms. These can be of a delusional nature, for example, "I should kill my self because it is Tuesday and Tuesday is the day of all deaths." or even present as hallucinations of hearing voices advocating suicide.

The physical symptoms of a depressive episode include a change in sleep patterns (usually insomnia, although oversleeping is also seen), a change in appetite with weight loss or gain, and fatigue. There is, interestingly, a cultural component to the expression of depression. If the pervading culture stigmatizes the psychology of depression, a patient may have vague physical complaints instead of psychological complaints. For example, the presence of emotional disturbances in some cultures may be considered taboo, and consequently, a patient suffering from MDD might present with an upset stomach, muscle aches, and headaches, instead of complaints of feeling sad, removed, and tired. These complaints do not respond to treatment, unless that treatment is directed towards the underlying depression.

In contrast to the depressive episodes associated with the conditions listed above, MDD is thought to be an endogenous phenomenon; that is, it originates internally. The depression of MDD causes specific changes in brain function and a number of neurotransmitters and circuits are involved. Once an episode of MDD has arisen, the depression will run its course, approximately six months, even if a possible trigger for the depression is removed. This course of depression is specific for MDD, and differences its depression from other causes of depression.

The obvious detriment of MDD is a miserable existence (morbidity) and the real risk of suicide (mortality). Fortunately, MDD is a treatable illness and approximately ninety percent of patients will respond to treatment - of which there are many, different successful options.