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 jinns, 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.

According to some religions, all the affairs of the universe 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. Some are potentially of the malignant type, to be propitiated by an appeal to knowledge of the supernatural. Traditional Korean belief posits that countless demons inhabit the natural world; they fill household objects and are present in all locations. By the thousands, they accompany travellers, seeking them out from their places in the elements.

Greek philosophers such as Porphyry of Tyre (who claimed influence from Platonism), as well as 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.

Not all spirits across all cultures are considered malevolent. In Central Africa, the Mpongwe believe in local spirits, just as the Inuit do; but they are regarded as inoffensive in the main. Passers-by must make some nominal offering as they near the spirits' residence. The occasional mischievous act, such as the throwing down of a tree on a passer-by, is believed by the natives to be perpetuated by the class of spirits known as Ombuiri.

Many spirits, especially those regarding natural processes, are often considered neutral or benevolent; ancient European peasant fears of the corn-spirit would crop up during irritation, as a result of the farmer infringing on the domain of said spirit, and taking his property by cutting the corn; similarly, there is no reason why the less significant pantheon should be regarded as malevolent, and historical evidence has shown that the Petara of the Dyaks are viewed as invisible guardians of mankind rather than hostile malefactors.

Demons are generally classified as spirits which are believed to enter into relations with the human race. As such the term includes:

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 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 ancient Mesopotamians believed that the underworld (Kur) was home to many demons, which are sometimes referred to as "offspring of arali". These demons could sometimes leave the underworld and terrorize mortals on earth. One class of demons that were believed to reside in the underworld were known as galla; their primary purpose appears to have been to drag unfortunate mortals back to Kur. They are frequently referenced in magical texts, and some texts describe them as being seven in number. Several extant poems describe the galla dragging the god Dumuzid into the underworld. Like other demons, however, galla could also be benevolent and, in a hymn from King Gudea of Lagash (c. 2144 – 2124 BC), a minor god named Ig-alima is described as "the great galla of Girsu". Demons had no cult in Mesopotamian religious practice since demons "know no food, know no drink, eat no flour offering and drink no libation."