The most common and destructive rats in the United States are called "Old World Rats" and were introduced accidentally in the 17th and 18th centuries by explorers and colonists. The Norway or brown rat is grayish-brown above and grayish below, and grows to 12 to 18 inches in length. Its tail, which is scaly, is about as long as its body. Black rats, which are found primarily in coastal regions of the U.S., are usually not black, but dark gray or brownish above and lighter gray below. They grow to 18 inches in length with a tail longer than that of the Norway rat. Signs of rats include dirty smudges around holes in walls, damaged goods, and tunnels near food supplies.

Rats have coarse fur, large ears, and pointed snouts. The source of much of rats' destructiveness is their powerful teeth, which grow continuously and must be filed down by constant gnawing. In fact, they have been known to gnaw through wooden boards, concrete and steel to get at food. Rats are omnivorous, but most often search out stores of grain. A colony of rats can do a great deal of damage to stored food, but they also may attack domestic animals, poultry and occasionally humans. In addition to their destructive feeding habits, rats harbor fleas, which can transmit diseases such as plague and typhus fever. The poisons used to control these pests can be dangerous and rats tend to be suspicious of baited food. Usually the best solution to an infestation is to contact a professional pest control operator who can safely eliminate the problem.