Mood is a constant affective background to all of conscious life. It fluctuates between pleasurable and energizing on the one hand, and depressive and enervating on the other. Mood responds to incident environmental influences and to internal, instinctual influences, but it also exhibits rhythmic fluctuations that influence behavior. Mood dysregulation creates affect disease, and the therapeutic agents that one uses to treat affect disease do so by controlling mood. Psychopharmacologic experience helps us to understand the principles of mood regulation and its effect on normal and abnormal behavior. Mood controls psychodynamics rather than vice versa. Important social issues such as apocalypticism and fundamentalism are ultimately mood driven. Mood is a significant psychic parameter that correlates with the many neuroscientific processes that are coming to light.