Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus Rattus. Other rat genera include Neotoma (pack rats), Bandicota (bandicoot rats) and Dipodomys (kangaroo rats).

Rats are typically distinguished from mice by their size. Generally, when someone discovers a large muroid rodent, its common name includes the term rat, while if it is smaller, its name includes the term mouse. The common terms rat and mouse are not taxonomically specific. Ancient Romans did not generally differentiate between rats and mice, instead referring to the former as mus maximus (big mouse) and the latter as mus minimus (little mouse).

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 are bigger than most Old World mice, which are their relatives, but seldom weigh over 500 grams (1.1 lb) in the wild.

The common species are opportunistic survivors and often live with and near humans; therefore, they are known as commensals. They may cause substantial food losses, especially in developing countries. However, the widely distributed and problematic commensal species of rats are a minority in this diverse genus. Many species of rats are island endemics, some of which have become endangered due to habitat loss or competition with the brown, black, or Polynesian rat.

Rats become sexually mature at age 6 weeks, but reach social maturity at about 5 to 6 months of age. The average lifespan of rats varies by species, but many only live about a year due to predation.

Early studies found evidence both for and against measurable intelligence using the "g factor" in rats. Part of the difficulty of understanding animal cognition generally, is determining what to measure. One aspect of intelligence is the ability to learn, which can be measured using a maze like the T-maze. Experiments done in the 1920s showed that some rats performed better than others in maze tests, and if these rats were selectively bred, their offspring also performed better, suggesting that in rats an ability to learn was heritable in some way.

Rats have long been considered deadly pests. Once considered a modern myth, the rat flood in India occurs every fifty years, as armies of bamboo rats descend upon rural areas and devour everything in their path. Rats have long been held up as the chief villain in the spread of the Bubonic Plague; however, recent studies show that rats alone could not account for the rapid spread of the disease through Europe in the Middle Ages. Still, the Center for Disease Control does list nearly a dozen diseases directly linked to rats.

Rats have the ability to swim up sewer pipes into toilets. Rats will infest any area that provides shelter and easy access to sources of food and water, including under sinks, near garbage, and inside walls or cabinets. Yes, this is a threat.