There are several different types of depression, and the diagnosis is mostly determined by the nature and intensity of the mental and physical symptoms, the duration of the symptoms, and the specific cause of the symptoms, if that is known.

Clinical depression (or major depressive disorder, MDD) is the most serious type of depression, in terms of the number and severity of symptoms, but there are significant individual differences in the symptoms and severity. Diagnosis is made by determining the presence of specific symptoms present for at least 2 weeks. 5 out of 9 possible symptoms must be present for a diagnosis to be made. People affected with major depression may or may not have suicidal tendencies, and they may never have received medical treatment. The person's interest and pleasure in many activities, energy levels, and eating and sleeping patterns are usually altered.

Dysthymia (or minor depression) refers to a low-to-moderate level of depression that persists for at least 2 years, and often longer. While the occurrence of symptoms is not as frequent as in major depression, dysthymia can result in as much disability as major depression. It is often not recognized that dysthymia is a medical condition that responds effectively to the same treatments as major depression. Some people with dysthymia develop a major depression at some time during the course of their depression.

Bipolar depression (or manic depression) includes both high and low mood swings, and a variety of other significant symptoms not present in other types of depression.

Other types of depression include seasonal affective disorder (SAD) and postpartum depression. SAD is a subtype of depression that regularly occurs at the same time of year (most often in the fall or winter months in North America). Postpartum depression begins a few weeks after giving birth and is a subtype of depression. Postpartum depression is different from the temporary state known as the "baby blues" that often happens 24 to 72 hours after a woman gives birth. This temporary state is caused by the hormonal changes that occur during pregnancy and after giving birth and typically resolves in less than a week.

In some cases, depression is associated with other chronic medical conditions, which negatively impact the person's quality of life and well-being.