An increasing number of data has been recently focused on recognizing pathological grief reactions and on the distinction from physiological processes. Particularly, several studies have supported Complicated Grief (CG) as an independent disorder, in order to define the failure of spontaneous physiological mourning resolution. Upon these studies, the latest edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) with the name of "Persistent Complex Bereavement Disorder". This article aims at retracing the history of clinical research about the physiological and psychopathological processes related to grief and aims at presenting the main scientific studies that have described the features of the CG defining it as a distinct disorder. Similarities and distinctions among CG and Depression, Posttraumatic Stress Disorder and Adjustment Disorder were also reported.