Although purring is commonly associated with felids, other animals such as raccoons purr. Other purring animals are rabbits, squirrels, guinea pigs, tapirs, cockroaches,ring-tailed lemurs, elephants, and gorillas while eating.

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, cats of the Panthera genus (Tiger, Lion, Jaguar and Leopard) also purr, but only when exhaling. 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.

Viverrids (civet, mongoose, genet) and hyaenas also purr.

No one actually knows why cats purr, although 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 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.