Derealization is a syndrome of mental disorder in which the environment seems dull, gray, “not alive”, “like in a photo”, devoid of color, movement. With severe derealization, doubts appear in the reality of the surrounding world. Derealization is often combined with depersonalization (see). The derealization also includes special states of the “never seen” and “already seen”, in which the familiar environment seems unfamiliar, alien, or, conversely, unfamiliar, the first seen situation seems familiar, already seen before. Derealization occurs in a variety of mental illnesses.