The present state of knowledge about depersonalization (DP) and derealization (DR) is reviewed with respect to classification, epidemiology, etiology, and therapy. Mild and transient DP-DR are considered to be common phenomena. The prevalence of depersonalization-derealization disorder (DP-DR-D) is estimated to be approx. 1-2% of the general population in the Western hemisphere. DP-DR-D is probably severely underdiagnosed. DP-DR-D is strongly associated with depression and anxiety disorders. It is suggested that symptoms of DP-DR indicate disease severity and negatively predict therapy outcome. Neurobiological and psychological models have shown that a disordered body schema and emotional and autonomic blunting are essential components of the disorder. Despite the frequency of DPDR and its clinical relevance, there is a considerable lack of empirical research on DP-DR with respect to the health-care situation of depersonalized patients and with regard to treatment options.