The theoretical approaches for determining the amount and distribution of heat flow in pistons are discussed and their limitations pointed out. A simplified theoretical model of a cylindrical disk with specified boundary conditions is introduced as a help to under standing the frequently cross-linked relations hips. Experimental work is evaluated and an attempt is made to establish a wider range of applicable ground rules. Engine speed, charge temperature, combustion turbulence, and mean pressure level of the cycle are recognized as influential parameters. The heat balance of direct piston cooling is presented in relation to the total heat release for various engine cycles. The specific capacity of heat transfer from the piston by the coolant is discussed.