All divination stems from the belief that gods send meaningful messages. These messages were classified in a variety of intersecting ways: according to the character of signs through which the message was conveyed, and whether these signs were sent unasked or were actively sought; the time-frame to which a sign was taken to refer (future, present, past) and the content of the message itself (prediction, warning, prohibition, displeasure, approval); and, most importantly, whether the message pertained to the private or public sphere, the observation and interpretation of the latter category of signs forming part of Roman state religion.

The divine message was either intuitively conveyed or required interpretation. Cicero (Div. 1. 12) adopts the division of divination (elaborated by the Stoics, see Stoicism) into two classes, artificial (external) and natural (internal). The latter relied upon divine inspiration (instinctus, adflatus divinus), and was characteristic of prophets (vaticinantes) and dreamers (somniantes). The former was based on art (ars) and knowledge (scientia). To this category belonged the observation of birds, celestial signs, entrails, unusual phenomena, also astrology and divination from lots. But inspired utterances (see Sibyl) and dreams also required interpretation.

The Roman state employed three groups of divinatory experts: the augures (augurs), the board of priests for the performance of sacred rites (see quindecimviri sacris faciundis), who were in charge of the Sibylline books (see Sibyl), and the haruspices. The first two were the official state priests; the haruspices were summoned as needed. Their special province was the observation of the entrails of sacrificial victims (haruspicy or extispicy), especially the liver (hepatoscopy). Both the augurs and haruspices observed and interpreted the avian and celestial signs (particularly fulmina and tonitrua, lightning and thunder), but they treated them differently. For the augurs they were the auspices expressing divine permission or prohibition concerning a specific act; they were indicative of the future only in so far as faulty auspices, and especially wilful disregard of auspices, might cause divine anger (which, however, could manifest itself in a variety of unpredictable ways). But for the haruspices (and also for the non-Roman augurs) the very same signs could be indications of specific future happenings.

All signs were either solicited or unsolicited. The latter could function either as unsolicited auspicia oblativa or as prodigies. The former referred solely to a concrete undertaking, the prodigies on the other hand to the state of the republic. They were indications that the normal relationship with the deity, the ‘peace of the gods’ (pax deum), was disturbed. Particularly potent were unusual occurrences (monstra, ostenta, Cic. Div. 1. 93). In the case of adverse auspices the action in question was to be abandoned; in the case of prodigies it was imperative to find out the cause of divine displeasure (this task often fell to the haruspices) and to perform various ceremonies of appeasement (procuratio). See portents.

The Roman state did not officially employ astrologers (occasionally they were even banned from Rome) or dream-interpreters (coniectores somniorum), but their services were sought by many, including the emperors (see astrology). Predictions were made also from involuntary motions, sneezing, and from lifeless objects, particularly from (inscribed) lots (sortes) drawn from a receptacle and interpreted by the sortilegi, with centres at Praeneste, Tibur, Antium, and the fountains of Aponus near Patavium and of Clitumnus in Umbria. Also poets were so used, particularly Virgil (sortes Vergilianae).

Popular divination was often scorned as charlatanry (by Ennius, Cicero, and the elder Cato (M. Porcius Cato (1): a haruspex could not but laugh on meeting another haruspex), and the government was particularly suspicious of astrologers and inspired prophets. In the Christian empire all forms of divination were prohibited and persecuted, though never eradicated.