The term "purring" has been used liberally in literature, and it has been claimed that viverrids (civet, mongoose, genet), bears, badgers, hyaenas (et cetera) purr. Other animals that have been said to purr are rabbits, squirrels, guinea pigs, tapirs, ring-tailed lemurs, elephants,, raccoons and gorillas while eating. However, using a strict definition of purring that continuous sound production must alternate between pulmonic egressive and ingressive airstream (and usually go on for minutes), Peters (2002), in an exhaustive review of the scientific literature, reached the conclusion that until then only ‘purring cats’ (Felidae) and two species of genets, Genetta tigrina, and most likely also Genetta genetta, had been documented to purr.

Although purring is a universally recognized phenomenon, the mechanism by which cats purr is elusive. This is partly because the cat has no unique anatomical feature that is clearly responsible for the sound.

One hypothesis, backed by electromyographic studies, is that cats produce the purring noise by using the vocal folds and/or the muscles of the larynx to alternately dilate and constrict the glottis rapidly, causing air vibrations during inhalation and exhalation. Combined with the steady inhalation and exhalation of air as the cat breathes, a purring noise is produced with strong harmonics. Purring is sometimes accompanied by other sounds, though this varies from cat to cat; in the audio samples that accompany this article, the first cat is only purring, while the vocal production of the second cat contains low level outbursts sometimes characterized as "lurps" or "yowps".

It was, until recent times, believed that only the cats of the Felis genus could purr. However, felids of the Panthera genus (Tiger, Lion, Jaguar and Leopard) also produce sounds similar to purring, but only when exhaling. The subdivision of the Felidae into ‘purring cats’ on the one hand and ‘roaring cats ’ (i.e. non-purring) on the other, originally goes back to Owen (1834/1835) and was definitely introduced by Pocock (1916), based on a difference in hyoid anatomy. The ‘roaring cats’ (lion, Panthera leo; tiger, P. tigris; jaguar, P. onca; leopard, P. pardus) have an incompletely ossified hyoid, which according to this theory, enables them to roar but not to purr. On the other hand, the snow leopard (Uncia uncia, or P. uncia), as the fifth felid species with an incompletely ossified hyoid, purrs (Hemmer, 1972). All remaining species of the family Felidae (‘purring cats’) have a completely ossified hyoid which enables them to purr but not to roar. However, Weissengruber et al. (2002) argued that the ability of a cat species to purr is not affected by the anatomy of its hyoid, i.e. whether it is fully ossified or has a ligamentous epihyoid, and that, based on a technical acoustic definition of roaring, the presence of this vocalization type depends on specific characteristics of the vocal folds and an elongated vocal tract, the latter rendered possible by an incompletely ossified hyoid.

Examples of a purring cheetah and a purring serval can be heard on Robert Eklund's Ingressive Speech website or on Robert Eklund's Wildlife page (see External links below). Further examples of a purring cheetah, a purring domestic cat and a purring genetta can be heard on Robert Eklund's purring site http://purring.org, where also film clips of a purring cheetah and film clips of purring waveforms and spectrograms are found. A purring bibliography is also provided.

No one actually knows why cats purr but the following reasons are speculated:

Cats often purr when being petted, becoming relaxed, or when eating. Female cats are known to sometimes purr while giving birth. Domestic cats have been reported to purr when injured, sick, in pain or dying. Purring may have developed as a signaling mechanism between mother cats and nursing kittens. One theory is that it is not a sign of showing relaxation or content, but an attempt at "friendship" or a signal of "specific intent". For example, when a cat is nervous and cannot escape the situation (at a veterinarian perhaps), its purr may serve as an attempt to avoid being hurt. German ethologist and cat behaviorist Paul Leyhausen interprets it as a signal that the animal is not posing a threat.

Scientists at the University of Sussex showed in 2009 that purring, or some purring, seems to be a way for domesticated cats to signal their owners for food. According to Dr. Karen McComb and her team, purring in the "about to be fed" context has a high-frequency component not ordinarily present. Humans report feeling an urgency to investigate and satisfy the cat's needs; to wit: "feed me." However, this variety of purring seems to be found only in cats in a one-on-one relationship with their caretakers. Another theory states that purring triggers a cat's brain to release a hormone[clarification needed] which helps them in relaxing and acts as a pain killer. This may be a reason why cats purr when distressed or in labour. This "soliciting purr" is different from a cat's normal purring.

Another theory holds that purring is beneficial to cats' health. The vibrations are thought to promote bone healing and to have the potential to dislodge hairballs from the pyloric valve.

Roger A. Caras opines that cats purr in response to profound emotions.