This book presents new and important research from around the world on depression which is a serious medical illness affecting 5 to 8 percent of the adult population in a given year. Unlike normal emotional experiences of sadness, loss, or passing mood states, major depression is persistent and can significantly interfere with an individual's thoughts, behaviour, mood, activity, and physical health. Among all medical illnesses, major depression is the leading cause of disability in the U.S. and many other developed countries. Depression occurs twice as frequently in women as in men, for reasons that are not fully understood. More than half of those who experience a single episode of depression will continue to have episodes that occur as frequently as once or even twice a year. Without treatment, the frequency of depressive illness as well as the severity of symptoms tends to increase over time. Left untreated, depression can lead to suicide.