Mood disorders are psychological disturbances characterized by intense emotional experiences of depression, mania, or both depression and mania. Mood disorders are remarkably diverse in their features but they share a focus on an excessive mood that colors and distorts the way the person thinks and views the world, accompanied by marked changes in movement, activity, energy, sleep, appetite, and motivation for engagement in the world. Depressive disorders are so frequent that they have been called the common cold of psychological disorders but, unlike the common cold, their consequences can be profoundly distressing, disruptive, and impairing. Included within this chapter will be current research and the prevailing theories regarding the phenomenology, etiology, and pathology of unipolar depressive disorders (e.g., major depressive disorder and dysthymia) and bipolar mood disorders (e.g., bipolar I, bipolar II, and cyclothymia).