A new disorder, prolonged grief disorder (PGD), was defined in the final version of the Internal Statistical Classification of Disease and Related Health Problems, 11th Edition (ICD-11) on July 2018. People with PGD keep intensely longing for the deceased or remain preoccupied with the deceased for a long time and can no longer stand their own life. PGD is included in the category of stress response syndromes and is regarded as a stress-related disorder, i. e., it is mainly considered to be a psychogenic or neurotic disorder.
 On the other hand, mental disorders related to bereavement have historically been referred to from widespread pathological viewpoints; most of them are compared to depression, melancholy, and hysterical psychosis. Some classical descriptions of depression thus far have shown that the pathogenic features of disorders related to bereavement range from bereavement reactions to complicated reactions, and even to melancholy or psychosis.
 We explore the process by which PGD appeared as a clinical classification, then next show how mental disorders related to bereavement have been recognized and classified in the field of clinical psychiatry, from melancholy in the seventeenth century to ideas of depression around Diagnostic and Statistical Manual of Mental Disorders, 3rd Edition (DSM-III), and post-traumatic stress disorder (PTSD). Lastly, we discuss psychoanalytic approaches to pathological grief that have helped in the development of the classical theory of PGD.
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