The existence of demons, while widely accepted, has always presented a theological difficulty. Since all things are ultimately the creation of the one God, the question of why should evil spirits exist has greatly exercised Jewish thought. Apocalyptic literature offers the first attempt to explain their existence in a monotheistic context by claiming demons are really fallen angels. Rabbinic literature provides the first extensive source for Jewish demonology, though the information is scattered though many sources. In it, several explanations for the existence of demons are offered. They are a creation of the twilight of the sixth day (Pirkei Avot 5.6). Abraham ibn Ezra described demons as a product of the interaction of sunlight with smoke and vapor which then clings to the body, causing illness (Sefer ha-Atzmim).
Demons cannot procreate on their own, so they used semen from Adam in order to make more of their own kind (B.T. Ervuin 18b; Zohar). An elaboration on this tradition is that Lilith, the first woman, having transformed herself into a witch-demon using the Tetragrammaton, takes the nocturnal emissions of men she seduces to procreate more demons (Alef-Bet ben Sira). In kabbalistic thought the demonic is a necessary part of creation, a product of the sitra achra, the "other side" of the divine emanations in the material universe.
Demons occupy an intermediate place between mortals and angels. According to Chagigah 16a, they resemble angels in three ways: They have wings, they can fly throughout the universe, and they hear what transpires in heaven. They also resemble mortals in that they procreate, eat, and die. They are always invisible, except under special conditions.
The malevolent effects of demons are many: they cause illness and death, especially for the vulnerable (children, women in childbirth); they trouble and deceive the mind, and cause contention in the community of mortals.
Tractate Berachot has perhaps the most information on demons of any part of the Talmud. There we learn that demons tend to dwell in the wilderness, in ruins, and in other places not by frequented by people (Isa. 13:21). It also describes a "diagnostic" ritual for detecting the presence of the demonic: Ashes spread around one's bed at nighttime will reveal demon tracks in the morning, and demons can be rendered visible by grinding up the ashes of a black cat's afterbirth and then sprinkle the powder in one's eyes.
The appearance of demons varies, but is always terrible. In keeping with Ancient Near Eastern beliefs about evil spirits, demons have bird talons for feet in addition to wings. At night, demons can appear in human form (Meg. 3a).
Demonic power waxes and wanes according to the time of day, the week, the seasons, meteorological conditions, topographical features, and other natural factors (Yalkut Chadash, Keshafim 56; Numbers R. 12:3; Pes. 3a-b, 112a; Shab. 67a). The informed can use this information to minimize their threat.
Around human habitations, they frequent rooftops, outhouses, and drainage gutters. Strangely, demonic forces are attracted to synagogues. The Angel of Death, for example, is said to keep his tools there. Stories of Sages doing night battles with demons in the synagogue appear in Jewish tales across time.
Prominent demons have names, usually derived from their particular power; Reshef, for example, means "pestilence." Some demons, like Samael, have theophonic names, like angels. Occasionally demons can have surprisingly mundane names, like "Joseph." The name Lilith means either "air" (Akkadian) or "night" (Hebrew) and has its roots in Mesopotamian aerial spirits called "lilu."
Reciting certain psalms has an atropopaic effect against evil spirits (Pss. 29: 91; 121), as do other key verses of Scripture (Num. 7:4-6). Magical phrases have also been recorded to combat their malevolent effects (Pes. 100a; 112a). The bells on the skirt of the High Priest evidently drove them away. Drinking water only from white containers turns away night demons (Pesachim 3a). Bercahot 5a credits ritual objects such as mezuzah, tefillin, and ritual fringes with warding off evil spirits. The Jews of Mesopotamia additionally protected their homes with Demon Bowls and Incantation Bowls. Temporary protection can be gotten through the use of magic circles. Amulets of near infinite variety have been created across Jewish history. Demons can be bribed with food or money, or frightened off with shofar blasts, unpleasant smells, or spitting. Guardian angels are the best defense, and are acquired every time one performs a mitzvah (Ex. R. 32).
Intriguingly, a mortal can work beneficently with demons, if one knows the rituals of power to control them. Asmodeus, the king of Demons, was co-opted by Solomon to good ends. Demons can be turned against other demons (Lev. R. 24). Sometimes the demon will do so willingly (Pes. 106a), but usually this involves controlling the demons magically and forcing the captured spirit to do the will of the adept. R. Eliezer of Metz (12th Century), permitted the use of demons in spells and amulets, writing, "Invoking the demons to do one's will is permitted" for what difference is there between invoking demons and angels?" At the same time, anything that smacks of demon veneration or worship, such as making offerings or burning incense to a demon, is expressly forbidden (Sanh. 65b).