In Jungian psychology, the shadow is a part of the unconscious mind which is mysterious and often disagreeable to the conscious mind, but which is also relatively close to the conscious mind. It may be (in part) one's original self, which is superseded during early childhood by the conscious mind; afterwards it comes to contain thoughts that are repressed by the conscious mind. The shadow is instinctive and irrational, but is not necessarily evil even when it might appear to be so. It can be both ruthless in conflict and empathetic in friendship. It is important as a source of hunches, for understanding of one's own more inexplicable actions and attitudes (and of others' reactions), and for learning how to cope with the more problematic or troubling aspects of one's personality.

The shadow may appear in dreams and visions in various forms, often as a feared or despised person or being, and may act either as an adversary or as a friend. It typically has the same apparent gender as one's persona. It is possible that it might tend to appear with dark skin to a person of any race, since it represents an old ancestral aspect of the mind. The shadow's appearance and role depend greatly on individual idiosyncrasies, because the shadow develops in the individual's mind rather than simply being inherited in the collective unconscious.

Interactions with the shadow in dreams may shed light on one's state of mind. A disagreement with the shadow may indicate that one is coping with conflicting desires or intentions. Friendship with a despised shadow may mean that one has an unacknowledged resemblance to whatever one hates about that character. These examples refer to just two of many possible roles that the shadow may adopt, and are not general guides to interpretation. Also, it can be difficult to identify characters in dreams, so that a character who seems at first to be a shadow might represent some other complex instead.

Jung has also made mention of there being more than one layer making up the shadow. The top layer is the rationally explicable unconscious. It contains material which has been made unconscious artificially; that is, it is made up of elements of one's personal experiences. Underneath this layer, however, is an absolute unconscious that has nothing to do with personal experiences. Jung described this bottom layer as "a psychic activity which goes on independently of the conscious mind and is not dependent even on the upper layers of the unconscious - untouched, and perhaps untouchable - by personal experience" (Campbell, 1971). This bottom layer of the shadow is also what Jung referred to as the collective unconscious.

According to Jung, the shadow sometimes takes over a person's actions, especially when the conscious mind is shocked, confused, or paralyzed by indecision.

The shadow might be the basis of the rank of Corax (raven) in the ancient religion of Mithraism.