This chapter presents the hopelessness theory of depression (Abramson, Metalsky, & Alloy, 1 989), a revision of the 1 978 reformulated theory of helplessness and depression (Abramson, Sel igman, & Teasdale, 1 978). Complementing clinical and taxonometric approaches, this theory represents a theory-based approach to the classification of a subset of depressive disorders that is process-oriented rather than symptom-oriented (see also Seligman, 1 978). In essence, the hopelessness theory postulates the existence in nature of an as yet unidentified subtype of depression, hopelessness depression, that may be lurking among the various disorders currently called depression. We describe the hypothesized cause, symp toms, course, therapy, and prevention of hopelessness depression as well as its relation to other types of depression and nondepression. In addition, we discuss how to search for hopelessness depression to see if it exists in nature and conforms to its theoretical description.