FOOD AND SHELTER are as essential to rats as to other animals, and the removal of these offers a practical means of permanent rat control. The number of rats on premises and the extent of their destructiveness are usually in direct proportion to the available food supply and to the shelter afforded. Rat proofing in the broadest sense embraces not only the exclusion of rats from buildings of all types but also the elimination of their hiding and nesting places and cutting off their food supply. Through open doors and in other ways, rats may frequently gain access to structures that are otherwise rat proof, but they can not persist there unless they find safe retreats and food. When rat proofing becomes the regular practice the rat problem will have been largely solved.