There are two principal concepts that often confuse people that are not familiar with cooling towers and how they operate. One, the cooling tower is a part of a bigger system; It does not set the heat load; the paper will explain how the tower interacts with the system. Two, the cooling tower and its components work as a system. The paper will explain how changes to one part of the tower can affect all the other parts of the tower: Such as selecting the economic size of the tower, affect of changing fill dimensions and type, heights, fan or stack size or type, inlet heights, location of louvers, nozzle type, choice of materials, temperature limits, definition of "tough" and "easy duties", and many more. It will define normal limits for air velocities throughout the tower, water loadings, fan power, and other good practice limitations. It will explain how pressure drop changes performance. It will discuss aftermarket changes that make things worse, not better. Comments will be general, not specific to any one product or type.