There are many theories about how cats purr. Probably the most widely accepted is that the sound and vibrations of the purr originate in the larynx (voice box) and are caused by the rapid opening and closing of structures called vocal folds. The vocal folds move in tandem with breathing, which enhances the sound and vibrations. The vocal tract directs the sound and vibration from the vocal folds out the mouth and nose, causing the chest, throat, and even other surfaces of the body to vibrate.

Apparently, purring is voluntary. Cats do not purr when in the deep stages of sleep. The Cheetah and Ocelot are two known wildcats that also make purr-like sounds.

We humans believe that when we hold or pet our cats, when the cat purrs, we feel a bond with our cats. We also belive that a cat is "happy" or "content" when it purrs. Purring may indeed be a way to communicate a bond. Kittens begin to purr when they are two days old, and the purring probably allows them to communicate with their mothers so that she knows they are all right and well fed.

Cats tend to treat us like giant mother cats, by purring in our presence, it's a vestige of the mother-kitten bond. Cats may also knead their paws against us while purring, as kittens this stimulates the mothers' milk. Since no one has ever caught a cat purring to itself, it appears to be done only in social situations.