Major Depressive Disorder (MDD) is very common in the United States and worldwide. In fact, studies have demonstrated that about approximately 12% of men and 25% of women will experience a MDD episode in their lifetime so severe that it impairs their ability to function. At any given time, it is estimated that about 5-6%, or one in twenty persons, is suffering from MDD. That translates into 18 million Americans suffering from MDD at any given time. The financial burden to society of MDD (lost productivity, cost of treatment) is staggering: it is estimated that MDD costs approximately $40 billion dollars per year in the United States. Severe MDD is costly in loss of life: it is estimated that 12-15% of individuals suffering from severe MDD commit suicide.