Demonology is the study of demons within religious belief and myth. Depending on context, it can refer to studies within theology, religious doctrine, or occultism. In many faiths, it concerns the study of a hierarchy of demons. Demons may be nonhuman, separable souls, or discarnate spirits which have never inhabited a body. A sharp distinction is often drawn between these two classes, notably by the Melanesians, several African groups, and others. The Islamic jinn, for example, are not reducible to modified human souls. At the same time these classes are frequently conceived as producing identical results, e.g. diseases.

A demon is a malevolent supernatural entity. Historically, belief in demons, or stories about demons, occurs in folklore, mythology, religion, and literature; these beliefs are reflected in media including comics, fiction, film, television, and video games. Belief in demons probably goes back to the Paleolithic age, stemming from humanity's fear of the unknown, the strange and the horrific. In ancient Near Eastern religions and in the Abrahamic religions, including early Judaism and ancient-medieval Christian demonology, a demon is considered a harmful spiritual entity that may cause demonic possession, calling for an exorcism. Large portions of Jewish demonology, a key influence on Christianity and Islam, originated from a later form of Zoroastrianism, and was transferred to Judaism during the Persian era.