Per the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), depressive disorders include disruptive mood dysregulation disorder, major depressive disorder (including major depressive episode), persistent depressive disorder (dysthymia), premenstrual dysphoric disorder, substance- / medication-induced depressive disorder, depressive disorder due to another medical condition, other specified depressive disorder, and unspecified depressive disorder.

Postpartum depression is classified as major depressive disorder with postpartum onset. It can last weeks to months. Perinatal depression includes prenatal depression, "baby blues," and postpartum depression (postpartum psychosis involves abrupt onset of psychotic symptoms; although relatively rare, it is a medical emergency).

Seasonal affective disorder is a variant depressive mood disorder characterized by symptoms of depression occurring during the winter and fall and resolving during the spring and summer. It is thought that shorter days and decreased exposure to natural sunlight are contributing factors. It can be treated with conventional depression treatment. Additionally, treatment with light box exposure is effective in improving mood when depressive symptoms cycle during the fall and winter months. 

The common feature of all of these disorders is the presence of sad, empty, or irritable mood, accompanied by somatic and cognitive changes that significantly affect the individual's capacity to function. What differs among them are issues of duration, timing, or presumed etiology.

Differentiating major depressive disorder from bipolar disorder is important, as treatment with antidepressants may adversely affect a person with undiagnosed bipolar disorder.

Patients with these disorders may have an increased risk of various general medical conditions.