Poena cullei (from Latin 'penalty of the sack') under Roman law was a type of death penalty imposed on a subject who had been found guilty of parricide. The punishment consisted of being sewn up in a leather sack, sometimes with an assortment of live animals, and then being thrown into water. The punishment may have varied widely in its frequency and precise form during the Roman period. For example, the earliest fully documented case is from ca. 100 BCE, although scholars think the punishment may have developed about a century earlier (earlier than that, murderers, including parricides, would be handed over to the aggrieved family for punishment, rather than punishment being enacted by Roman state officials). Inclusion of live animals in the sack is only documented from Early Imperial times, and at the beginning, only snakes are mentioned. At the time of Emperor Hadrian (2nd century CE), the most well known form of the punishment was documented, where a cock, a dog, a monkey and a viper were inserted in the sack. However, at the time of Hadrian "poena cullei" was made into an optional form of punishment for parricides (the alternate being thrown to the beasts in the arena). During the 3rd century CE up to the accession of Emperor Constantine, "poena cullei" fell out of use; Constantine revived it, now with only serpents to be added in the sack. Well over 200 years later, Emperor Justinian reinstituted the punishment with the four animals, and "poena cullei" remained the statutory penalty for parricides within Byzantine law for the next 400 years, when it was replaced with the punishment for parricides to be burnt alive instead.

"Poena cullei" gained a revival of sorts in late medieval and early modern Germany, with late cases of being drowned in a sack along with live animals being documented from Saxony in the first half of the 18th century.

The 19th-century historian Theodor Mommsen compiled and described in detail the various elements that at one time or another have been asserted as elements within the ritualistic execution of a parricide during the Roman Era. The following paragraph is based on that description, it is "not" to be regarded as a static ritual that always was observed, but as a descriptive enumeration of elements gleaned from several sources written over a period of several centuries. Mommsen, for example, notes that the monkey hardly can have been an ancient element in the execution ritual.

The person was first whipped, or beaten, with "virgis sanguinis" ("blood-colored rods", probably), and his head was clad/covered in a bag made of a wolf's hide. On his feet were placed clogs, or wooden shoes, and he was then put into the "poena cullei", a sack made of ox-leather. Placed along with him into the sack was also an assortment of live animals, arguably the most famous combination being that of a serpent, a cock, a monkey and a dog. The sack was put on a cart, and the cart driven by black oxen to a running stream or to the sea. Then, the sack with its inhabitants was thrown into the water.

Other variations occur, and some of the Latin phrases have been interpreted differently. For example, in his early work De Inventione, Cicero says the criminal's mouth was covered by a leathern bag, rather than a wolf's hide. He also says the person was held in prison until the large sack was made ready, whereas at least one modern author believes the sack, "culleus", involved, would have been one of the large, very common sacks Romans transported wine in, so that such a sack would have been readily available. According to the same author, such a wine sack had a volume of .

Another point of contention concerns precisely how, and by what means, the individual was beaten. In his 1920 essay ""The Lex Pompeia and the Poena Cullei"", Max Radin observes that, as expiation, convicts were typically flogged until they bled (so some commentators translates the phrase to "beaten with rods till he bleeds"), but that it might very well be that the rods themselves were painted red. Radin also points to a third option, namely that the "rods" actually were some type of shrub, since it documented from other sources that whipping with some kinds of shrub was thought to be purifying in nature.

Pain is the main reason for visiting an emergency department in more than 50% of cases, and is present in 30% of family practice visits. Several epidemiological studies have reported widely varying prevalence rates for chronic pain, ranging from 12 to 80% of the population. It becomes more common as people approach death. A study of 4,703 patients found that 26% had pain in the last two years of life, increasing to 46% in the last month.

A survey of 6,636 children (0–18 years of age) found that, of the 5,424 respondents, 54% had experienced pain in the preceding three months. A quarter reported having experienced recurrent or continuous pain for three months or more, and a third of these reported frequent and intense pain. The intensity of chronic pain was higher for girls, and girls' reports of chronic pain increased markedly between ages 12 and 14.

The most reliable method for assessing pain in most humans is by asking a question: a person may report pain that cannot be detected by any known physiological measure. However, like infants, animals cannot answer questions about whether they feel pain; thus the defining criterion for pain in humans cannot be applied to them. Philosophers and scientists have responded to this difficulty in a variety of ways. René Descartes for example argued that animals lack consciousness and therefore do not experience pain and suffering in the way that humans do. Bernard Rollin of Colorado State University, the principal author of two U.S. federal laws regulating pain relief for animals, writes that researchers remained unsure into the 1980s as to whether animals experience pain, and that veterinarians trained in the U.S. before 1989 were simply taught to ignore animal pain. In his interactions with scientists and other veterinarians, he was regularly asked to "prove" that animals are conscious, and to provide "scientifically acceptable" grounds for claiming that they feel pain. Carbone writes that the view that animals feel pain differently is now a minority view. Academic reviews of the topic are more equivocal, noting that although the argument that animals have at least simple conscious thoughts and feelings has strong support, some critics continue to question how reliably animal mental states can be determined. The ability of invertebrate species of animals, such as insects, to feel pain and suffering is also unclear.

The presence of pain in an animal cannot be known for certain, but it can be inferred through physical and behavioral reactions. Specialists currently believe that all vertebrates can feel pain, and that certain invertebrates, like the octopus, may also. As for other animals, plants, or other entities, their ability to feel physical pain is at present a question beyond scientific reach, since no mechanism is known by which they could have such a feeling. In particular, there are no known nociceptors in groups such as plants, fungi, and most insects, except for instance in fruit flies.

In vertebrates, endogenous opioids are neuromodulators that moderate pain by interacting with opioid receptors. Opioids and opioid receptors occur naturally in crustaceans and, although at present no certain conclusion can be drawn, their presence indicates that lobsters may be able to experience pain. Opioids may mediate their pain in the same way as in vertebrates. Veterinary medicine uses, for actual or potential animal pain, the same analgesics and anesthetics as used in humans.