This paper addresses the fundamental heat transfer augmentation question of how to arrange a stack of parallel plates (e.g. fins of heat sink, printed circuit boards) in a free stream such that the thermal resistance between the stack and the stream is minimum. It is shown that the best way of positioning the plates relative to one another is by spacing them equidistantly. When the overall dimensions of the stack are specified, there is an optimal number of plates for minimum thermal resistance. The optimal number and minimum resistance are anticipated theoretically and correlated into compact formulas that agree with numerical and experimental results in the Re, range 10(2)-10(4). Finally, it is shown that a stack with more plates than the optimal number can be modeled more expediently as a porous block immersed in a free stream.