• Trouble sleeping or excessive sleeping
• A dramatic change in appetite, often with weight gain or loss
• Fatigue and lack of energy
• Feelings of worthlessness, self-hate, and inappropriate guilt
• Extreme difficulty concentrating
• Agitation, restlessness, and irritability
• Inactivity and withdrawal from usual activities, a loss of interest or pleasure in activities that were once enjoyed (such sex)
• Feelings of hopelessness and helplessness
• Thoughts of death or suicide

Depression can appear as anger and discouragement rather than feelings of hopelessness and helplessness. If depression is very severe, it may be accompanied by psychotic symptoms, such as hallucinations and delusions. These are usually consistent with the depressed mood, and may focus on themes of guilt, personal inadequacy, or disease.

Major depression is diagnosed if the person reports having 5 or more depressive symptoms for at least 2 weeks. Beck's Depression Scale Inventory or other screening tests for depression can be helpful in diagnosing depression.

Medical causes that can show symptoms of depression should also be ruled out before making the diagnosis of depression.