The object of the paper is to present the fundamental theory of natural-draught cooling towers in a form which can be applied by non-specialist engineers to the problems which arise in the design and operation of electricity generating stations.After a brief general discussion, the work of previous investigators is summarized. A convenient approximate treatment of the transfer process due to Merkel is expressed as a nomogram. The conditions under which the approximations lead to appreciable errors are defined in a form suitable for practical use.The variation of air rate through a natural-draught cooling tower with design and operating conditions is defined by an approximate equation. It is shown that routine design and performance data can be reduced to two dimensionless quantities between which correlation would be expected on theoretical grounds.By considering numerous test results in conjunction with the theoretical relationships it is shown that all tests on a given tower can be represented by a single number, termed the “performance coefficient” which is substantially independent of tower size and so offers a useful basis for comparison, interpolation and extrapolation between towers of comparable design.The application of the performance coefficient and a related quantity to the solution of practical problems is discussed and illustrated by worked examples.