Depression is a state of low mood and lack of interest that affects thoughts, behavior, feelings, and well-being. It has various causes like genetics, life stressors, medical illnesses, or chemical imbalances. There are two main types of depression: unipolar, seen in most cases and characterized by periods of sadness without mania; and bipolar disorder, seen in some and characterized by cycling between depression and mania. Current understanding suggests depression may be related to deficiencies in serotonin, norepinephrine, and dopamine in the brain. Treatments include antidepressant medications that target these neurotransmitters, electroconvulsive therapy, and alternative medicines.