This chapter examines the psychopathology of affective disorders. It distinguishes between the depressive episode, and manic episode. The history of the classification of affective disorders is one of controversy and confusion that is only partially resolved in the current editions of the International Classification Diseases (ICD-10) or the DSM-IV. Both ICD-10 and Statistical Manual of Mental Disorders (DSM)-IV classification allow that subcategories of depression may occur depending on whether psychotic symptoms are present. The causation of affective disorders is complicated but can be generally considered to reflect interplay between constitutional or biological factors and reaction to environmental insults. Evidence on the causes of affective disorders comes from a wide variety of sources. The overlap between bipolar disorder and schizophrenia presents perhaps the biggest challenge as molecular genetic studies continue to reveal polymorphisms in genes that appear to be associated with liability to each of these major Kraepelinian categories.