In this chapter, we will review pharmacological concepts underlying the use of drugs used to treat mood disorders, from depression, to mixed states, to mania. These agents have classically been called “antidepressants” and “mood stabilizers” but this terminology is now considered out of date and confusing since not all drugs classically called “antidepressants” are used to treat all forms of depression – especially not bipolar depression or depression with mixed features. Furthermore, many of the classic so-called “antidepressants” are also used to treat a whole range of disorders from anxiety disorders, to eating disorders, traumatic disorders, obsessive compulsive and impulsive disorders, pain, and beyond. Finally, many of the drugs used for psychosis and discussed extensively in Chapter 5 are used even more commonly to treat depression, unipolar, bipolar, and mixed depression, as well as mania, yet are not generally classed as “antidepressants” although they are certainly “drugs for depression.” To eliminate confusion about how to discuss categories of drugs, throughout this textbook we strive to utilize modern neuroscience-based nomenclature, where drugs are named for their pharmacological mechanism of action and not for their clinical indication.