1.A mood disorder, an emotional disorder 2.Females are more likely to experience depression, most prevalent in middle aged people 45-64, African American and Hispanic have a little higher prevalence also. 3.Environmental: trauma, divorce, unemployment, school, friends, finances 4.a. Biogenic amine theory: depression is associated with absolute or relative deficiency of neuronal catecholamines (serotonin, norepinephrine and dopamine (basis for all current medications)