The Norway Rat is also known as the house rat, brown rat, wharf rat, sewer rat, and water rat. It was first introduced into the United States by European settlers and trading ships around 1775. It is now the most widely distributed rat species in North America and can be found in all states.

The Norway rat is larger, stronger, more aggressive, and better adapted for producing young and surviving in colder climates than is the Roof rat or other rat species.

Identification of the Norway rat is simple. This rat variety has a stocky body, weighing between 12 and 16 ounces as an adult. The body fur is coarse and ranges from reddish to grayish brown, with buff-white underparts, but there are many color variations including the all black Norway rat. The nose is blunt, the ears are small and close-set and do not reach the eyes when pulled down. The tail is scaly and shorter than the combined length of the head and body.

Breeding periods for the Norway rat are normally in the spring and fall of the year, decreasing during the hot months of summer and the cold of winter. After mating and a gestation period of around 22 days, the mother gives birth to a litter consisting of from 8 to 12 pups. The young are naked and blind at birth and open their eyes in about 9 to 14 days; they are weaned 10 to 15 days after that. The pups are curious at about this stage and begin taking short exploratory trips around their surroundings and are taught by the mother how to find food, water, and how to hide from danger. The average female may have from 4 to 7 litters per year and may successfully wean 20 or more pups annually.

Norway rats live for more than three years in captivity, but in the wild, they live from 5 to 12 months. Predators and disease, severely curtail and influence average life span. Rats, like mice, are social animals and live in colonies. Thus, some of the mouse behavior is similar to rats, but there are important differences. The Norway rat is a ground-dwelling animal and frequently nests outdoors in underground burrows. On farms, they inhabit barns, silos, and other structures. In cities, this rat may spend its entire life inside urban buildings. Indoors, Norway rats prefer to nest around the lower floors of buildings, but when populations are large, it will also occupy attics, suspended ceilings, wall voids and crawl spaces.

Nests are built of almost any soft material, such as paper, cloth, leaves, and grass or hay and are often chewed into smaller pieces in order to make a loosely matted mass which can be better molded into a space that suits that particular mammal.