Major depressive disorder (MDD), commonly known as depression, is a medical illness that involves mood and behavior changes. It affects over 23 million US adults at some point in their lives. While there is no single identified cause of depression, it is thought to involve genetic and environmental factors interacting. Symptoms include persistent sadness, irritability, sleep problems, weight changes, loss of interest, and difficulty concentrating. Treatment options include antidepressant medications like SSRIs, SNRIs, NDRIs, and others, as well as occupational therapy which helps patients engage in meaningful activities.