High operating evaluations for cooling towers have led to new counterflow designs and changes in typical operating parameters. Experience has taught that such changes require detailed analysis. Extensive modeling and full-scale studies have defined limits for several parameters beyond which predictability of performance can suffer seriously. Precisely the economically optimum operating areas are affected. Evaluated price difference can be quite large between appropriately limited selections and optimum selections without regard to limits. Such differences have been demonstrated for both natural and mechanical draft. The largest potential impact occurs when difficult design conditions aggravate trends towards limits for natural draft.