According to the Integrated Taxonomic Information System (ITIS) there are more than 60 species of rat, which means they come in all sizes.

The best-known rat species are the black rat (Rattus rattus) and the brown rat (Rattus norvegicus). This group, generally known as the Old World rats or true rats, originated in Asia.

Rats use their tails to help them keep their balance, to communicate, and to keep their bodies at the right temperature.

A female rat typically births six litters a year consisting of 12 rat pups, although 5-10 pups is more common. Each pup reaches sexual maturity and begins breeding after 4-5 weeks, meaning that a population can grow exponentially. If left unchecked, a pair of rats can produce 488,281,250 pups in just three years.

Kangaroo rats obtain enough water from the metabolic oxidation of the seeds they eat to survive and don't need to drink water at all.

A rat can squeeze through a hole the size of a quarter, thanks to its collapsible skeleton. Its ribs are hinged at the spine and can fold down like an umbrella, which means that any hole that's big enough for a rat's head is big enough for the rest of him.

The Gambian pouched rat (Cricetomys gambianus) is among the largest rats in the world, growing up to about 0.9 metres (3 ft) long including the tail which makes up half of its total length.

Bubonic plague outbreaks are controlled by pest control and modern sanitation techniques. This disease uses fleas commonly found on rats as a vector to jump from animals to humans. The plague was the cause of the Black Death that swept through Asia, Europe, and Africa in the 14th century and killed an estimated 50 million people.

Fear of mice and rats is one of the most common specific phobias. It is sometimes referred to as musophobia (from the Greek word for "mouse") or murophobia (for the family Muridae that encompasses mice and rats).

All rats have a pair of upper and a pair of lower teeth called incisors. Unlike our teeth, these incisors don't have roots, and they never stop growing. If they didn't gnaw constantly to wear them down, these teeth would grow until they pierced the opposite jaw.