The Norway rat is heavy-bodied rodent with coarse tan, brown, gray, or dark gray fur and a lighter belly. The combined adult head and body length is 7 to 9 1/2 inches. The tail is about 6 to 8 inches. Adult Norway rats weigh about 7 to 18 ounces and are occasionally larger. The eyes are small, and the ears cannot be folded to cover the eyes. A Norway rat's nose is blunt in shape and the tail is naked and scaly in appearance. Rat droppings are large compared to those of mice, about 1/2 to 3/4 inch long and rounded on the ends. Rats are mainly nocturnal and tend to be very cautious, especially about new objects in their environment. Norway rats dig burrows outside buildings, along foundations, or under vegetation, but they also nest in basements and undisturbed storage or debris areas. A young rat can gain entry into a building through an opening the diameter of a quarter.

Norway rats reproduce quickly when food and water are abundant. Sexual maturity is reached at 8 to 12 weeks and development is about 23 days. Each litter contains an average of 7 to 8 pups but can be as large as 20 pups when food is plentiful. Pups are weaned by 3 to 4 weeks of age. A female Norway rat can have up to 12 litters per year. The actual number of weaned offspring is usually lower than this due to mortality. The life span of a Norway rat is about 5 to 12 months in the wild (cities or rural areas), but they can live far longer in captivity, as many people captive breed this species (called fancy rats) as pets.

Norway rats damage and destroy materials and property and cause structural damage by gnawing. They also feed on and contaminate stored foods because rodents urinate and defecate constantly as they move about. Norway rats carry and vector diseases, including murine typhus, leptospirosis, rat bite fever, and cow pox, and foodborne illnesses such as trichinosis and salmonella. Plague, which is often attributed to rats is as- sociated with the roof rat, is absent in the eastern United States. Parasites of rats that affect humans include fleas and tropical rat mites, both of which can become problems when rats die in buildings.