Demonology, branch of religion which deals with evil spirits or demons, as distinguished from Angelology, which deals with good spirits. In every religion the existence of evil spirits is recognized. The spirits or ghosts of the dead have generally been regarded with awe and fear (except in Christianity and Judaism) and worshiped or propitiated by the living; but they are to be distinguished from spirits which have never inhabited a human body, daimons or demons proper, which are frequently associated with the powers of nature.
The distinction between good and evil spirits depends more upon the sphere of their action than upon their nature. Fire and disease are evil when they attack the individual, but beneficent when invoked against his enemies. As the idea of God progressed so that of the evil forces opposed to Him developed too. In polytheistic religions, the many gods are balanced by a multitude of demons and spirits, each often having its especial sphere assigned to it. In monotheistic religions one God is usually opposed by one single evil spirit. Perfect dualism is found in Zoroastrianism, where every good is opposed by its evil, and Ormuzd is opposed by Ahriman. Similarly in Christianity and Old Testament Judaism, Jehovah is opposed by Satan, and the hosts of the good angels are opposed by the hosts of the fallen angels. Here, however, we find the evil definitely placed in subordination to the good, a fact which is most clearly exemplified in Job, where Satan appears as a servant of God, whose permission he must ask before tempting Job.

The gospels distinctly recognize the belief that disease was sometimes the result of possession by demons, and the early Church carried on Christ's method of expelling them. Exorcists long formed an active minor order, and the name still remains in the Roman Catholic Church. The common opinion of the early Church was that the gods of all heathen nations were evil spirits who had usurped the place of God, an idea which found its greatest expression in Milton's Paradise Lost. In the northern lands they were further discredited by being made ridiculous. The Satan of the miracle plays was a fool whom everyone might outwit, the ancestor of the Elizabethan clown.

The most elaborate system of demonology is the Muslim, which is largely derived from that of the popular Judaism shown in the apocalyptic books. Belief in the existence of evil spirits was very lively in the Middle Ages and sometimes took superstitious and heretical forms, as in the practice of satanism or witchcraft. Late examples of belief in witchcraft among the educated may be found in Burton and Sir Thomas Browne, and both Addison and Johnson refer to it. Popular belief has not yet died. The belief in vampires is chiefly found in Slavonic lands, but many references to the succubi who consort with men in their sleep, and incubi, who consort with women, may be found everywhere.