Norway rats are brownish or gray in color and can grow to lengths exceeding 18 inches, including the tail. They weigh between seven and twelve ounces on average, but can sometimes grow to more than a pound.

Norway rats often excavate burrows in the ground in fields, under sheds or buildings, at the base of trees, and under rocks. But they also make themselves at home in abandoned buildings, basements, garbage dumps, sewers, around garbage dumpsters, and pretty much anywhere else they can find food, water, and relative seclusion.

Norway rats are also very good swimmers, have good balance, can climb well when need be, and are comfortable on ledges, pipes, and other narrow horizontal surfaces.

Norway rats have excellent senses of hearing, smell, touch, and taste, which make it difficult to develop rodenticides that are palatable to them. Their vision, however, does not seem to be very sharp beyond a few feet, and they are believed to be color-blind.

Like most rodents, Norway rats have impressive reproductive potential. They reach sexual maturity at the age of three months, on average, and adult females come into heat every four or five days. The gestation period lasts about 23 days, and there is an average of six to twelve young in a litter. Females may have as many as six litters in a year, successfully weaning about 20 to 30 young every year, on average.