Many men have devoted their lives to a study of methods of rat control, and as a result countless preparations, devices, and contrivances are constantly being made available. Trapping, snaring, trailing, flooding, digging, hunting, ferreting, poisoning, and fumigating are employed, and rat limes, rat lures, rat repellents, and bacterial viruses are resorted to, and even antirat laws, local. State, and national, are constantly being passed in a world-wide effort to conquer this rodent. These have been important factors in keeping down the surplus, but all destructive agencies that have been used have utterly failed to reduce materially the total number of rats in the world. Rat proofing, however, is at last making definite headway against the age-old enemy of mankind, and it is upon this that the ultimate solution of the rat problem will depend. The destruction of rats for temporary relief and for keeping them under control in places where rat proofing is not possible or practicable will always be necessary, and knowledge of the best means of destroying rats is essential to any rat-control program. Information on poisoning, trapping, and other means of destroying rats is given in Farmers' Bulletin 1533, Rat Control. Permanent freedom from rats, however, should be the goal of everyone troubled with the pests and rat proofing offers the best means to this end. GENERAL PRINCIPLES OF RAT PROOFING Every separate structure presents its individual problem, but there are two general principles that apply in all cases and that should be kept in mind when the rat proofing of any building is being considered. First, the exterior of those parts of the structure accessible to rats, including porches or other appurtenances, must be constructed of materials resistant to the gnawing of rats, and all openings must be either permanently closed or protected with doors, gratings, or screens; second, the interior of the building must provide no dead spaces, such as double walls, spaces between ceilings and floors, staircases, and boxed-in piping, or any other places where a rat might find safe harborage, unless they are permanently sealed with impervious materials.