Demonology is the systematic study of demons or beliefs about demons. It is the branch of theology relating to supernatural beings who are not gods. It deals both with benevolent beings that have no circle of worshippers or so limited a circle as to be below the rank of gods, and with malevolent beings of all kinds. The original sense of "demon", from the time of Homer onward, was a benevolent being, but in English the name now holds connotations of malevolence. (In order to keep the distinction, when referring to the word in its original Greek meaning English uses the spelling "Daemon" or "Daimon".)

Demons, when regarded as spirits, may belong to either of the classes of spirits recognized by primitive animism; that is to say, they may be human, or non-human, 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 Arab jinn, for example, are not reducible to modified human souls; at the same time these classes are frequently conceived as producing identical results, diseases.

According to some societies, all the affairs of life are supposed to be under the control of spirits, each ruling a certain "element" or even object, and themselves in subjection to a greater spirit. For example, the Inuit are said to believe in spirits of the sea, earth and sky, the winds, the clouds and everything in nature. Every cove of the seashore, every point, every island and prominent rock has its guardian spirit. All are potentially of the malignant type, to be propitiated by an appeal to knowledge of the supernatural. Traditional Korean belief posits countless demons inhabit the natural world; they fill household objects and are present in all locations. By the thousands they accompany travelers, seeking them out from their places in the elements.

In ancient Babylon, demonology had an influence on even the most mundane elements of life, from petty annoyances to the emotions of love and hatred. The numerous demonic spirits were given charge over various parts of the human body, one for the head, one for the neck, and so on.

Greek philosophers such as Porphyry, who claimed influence from Platonism, and the fathers of the Christian Church, held that the world was pervaded with spirits, the latter of whom advanced the belief that demons received the worship directed at pagan gods.

Many religions and cultures believe, or once believed, that what is now known as soothsaying, was, or is, a form of physical contact with demons.

The ascription of malevolence to the world of spirits is by no means universal. In Central Africa, the Mpongwe believe in local spirits, just as do the Inuit; but they are regarded as inoffensive in the main. Passers-by must make some trifling offering as they near the spirits' place of abode; but it is only occasionally mischievous acts, such as the throwing down of a tree on a passer-by, are, in the view of the natives, perpetuated by the class of spirits known as Ombuiri. So too, many of the spirits especially concerned with the operations of nature are conceived as neutral or even benevolent; the European peasant fears the corn-spirit only when he irritates him by trenching on his domain and taking his property by cutting the corn; similarly, there is no reason why the more insignificant personages of the pantheon should be conceived as malevolent, and we find that the Petara of the Dyaks are far from indiscriminating and malignant, being viewed as invisible guardians of mankind.

Under the heading of demons are classified only such spirits as are believed to enter into relations with the human race; the term therefore includes: angels in the Judeo-Christian tradition that fell from grace,human souls regarded as genii or familiars, such as receive a cult (e.g., ancestor worship),ghosts or other malevolent revenants.

Excluded are souls conceived as inhabiting another world. Yet just as gods are not necessarily spiritual, demons may also be regarded as corporeal; vampires for example are sometimes described as human heads with appended entrails, which issue from the tomb to attack the living during the night watches. The so-called Spectre Huntsman of the Malay Peninsula is said to be a man who scours the firmament with his dogs, vainly seeking for what he could not find on Earth -a buck mouse-deer pregnant with male offspring; but he seems to be a living man; there is no statement that he ever died, nor yet that he is a spirit. The incubi and Succubi of the Middle Ages are sometimes regarded as spiritual beings; but they were held to give proof of their bodily existence, such as offspring (though often deformed). Belief in demons goes back many millennia. The Zoroastrian faith teaches that there are 3,333 Demons, some with specific dark responsibilities such as war, starvation, sickness, etc.

Demons are at times summoned to possess a body or they come to claim a payment from the necromancer.