Kzin are larger than Humans, a large male Kzin may be eight feet tall and weigh nearly 500 pounds. They are bipeds, standing erect on short legs. They have bright orange fur with individualized black markings, which most commonly appear on the face and hands. They are powerfully built, with thick limbs and torso. Although mammalian they do not have a mammal-like rib cage. Instead they have a latticework of bone struts encasing their torso, connected by cartilage. The head is quite cat-like in appearance, although with a large cranial bulge. Their eyes are strangely colored, with round pupils instead of cat-like vertical ones. Their fan-like ears fold like Chinese parasols. They have a naked, pink, rat-like tail. Kzin (their home planet) has a gravity of approximately 1.2 gee. However, the Kzinti colonized other worlds millennia before Humans did, so different Kzinti are adapted to different gravity levels. Kzinti and Human biochemistry are very similar; they can eat Humans without allergic reaction, and gain nourishment from the meat.

Female Kzinti are only semi-intelligent (or at least non-verbal) and are kept closely guarded in harems. They are never carried aboard Kzinti warships or intentionally exposed to the dangers of war. Kzinti females are in heat only when fertile, like most mammals but unlike Humans. It appears likely that Kzinti engage in sex only for reproduction, also unlike Humans.

Although it is never stated directly, it seems that the Kzinti are on average less intelligent than Humans. Aggressive and territorial, Kzinti have much more need of "elbow room" than do Humans. A Kzin "grin" is not a smile— the Kzin is baring his teeth for an attack. When one Kzin challenges another, he screams and leaps. When time permits they frequently perform grooming, such as brushing their fur. The Heroes' Tongue, the Kzinti language, sounds like cats fighting.

The Kzinti Empire is ruled by a feudal-style aristocracy. Those with high status have full names; those with low status have not earned a name, and are called by their profession. A moderate status earns a "partial name", for example "Chuft-Captain". Unlike Human aristocrats, even a Kzin born to a high-status family must earn his name. According to a Puppeteer (who may have a biased perspective), Kzinti refuse to perform menial work or practice animal husbandry. Therefore they must have slaves to perform these tasks, "or be barbarians roaming the forests for meat". A "conquest leader", leading the conquest of a planet, is entitled to one percent of its wealth for life. Kzinti use base eight numbers, perhaps indicating they have eight fingers.

Kzinti believe they should rule the galaxy, and until decisively defeated by Humans in the Man-Kzin Wars they conquered all intelligent species they encountered, enslaving the Kdatlyno and the Pierin, and apparently exterminating the Chunquen. Even after the Man-Kzin Wars were over, the Kzinti still owned Jotaki and Kdatlyno slaves. In times of peace, Humans sell foodstuffs to the Kzinti, buy metals from them, and employ them as games theorists.

It appears as a series of dots and commas. See our Kzinti Language Page.

When it made first contact with Humans circa 2366, the Kzinti Empire was much larger than Human space, and ruled several slave races. Over the course of four Wars with Men (Man-Kzin Wars), the Empire was greatly reduced. Two worlds were confiscated at the end of each war, and in 2850 the Kzinti numbers were less than an eighth of what they were when they first met Humans. The Covenants of Sasht, negotiated at the end of the fourth and last war in 2505, restricted the Kzinti and their spaceships to police weapons only. Worlds belonging to some or most of its slave races, including the Kdatlyno and Pierin, were freed during these wars. However, in 2658 some Kdatlyno legally remained Kzinti slaves. In 2645 the Kzinti Empire was still slightly larger than Human space.