Depression (major depressive disorder or clinical depression) is a common but serious mood disorder. It causes severe symptoms that affect how you feel, think, and handle daily activities, such as sleeping, eating, or working. Not everyone who is depressed experiences every symptom. Some people experience only a few symptoms while others may experience many. Depression, from mild to the most severe cases, can be treated. The earlier that treatment can begin, the more effective it is. How to Talk with Your Doctor about Depression