Major depressive disorder is a mental illness characterized by low mood, low self-esteem, and loss of interest in activities. It affects over 3% of Americans and can be disabling by impacting family, work, sleep, and eating habits. If left untreated, it may lead to higher risk of suicide and shorter life expectancy. Causes include genetic, biological, psychological, and social factors. Common treatments are medication, psychotherapy, and electroconvulsive therapy which have shown success in managing symptoms and prognosis.