FROM 25 to 35 per cent of the heat energy of the fuel inducted into the cylinders of an internal-combustion engine must be eliminated by the cooling system. As this waste requires the expenditure of energy, the devising of an efficient cooling system is imperative. The author, who is a leading American authority on engine problems, discusses the theory of liquid-cooling, gives heat-transfer and temperature-balance equations that must be satisfied and points out the three interrelated variable factors that must be incorporated in their most economical relation. The cooling system must be studied as a whole, rather than from the standpoint of any particular unit. Some commonly held beliefs regarding fans, fuel-pumps and oil-temperature control are controverted. Five elements necessary for an efficient cooling system are enumerated.