Though thermal considerations are one of the major limitations to the output obtainable from electrical machines, the thermal design of machines and their cooling circuits seems seldom to be accorded quite the attention that is normally directed to the electrical and magnetic circuits. There are various ways in which the designer of an electric motor handles the losses occurring in the different parts of the machine: generally, by dissipation from the surface, but also by conduction from areas that are difficult to cool adequately to those where this is easier to achieve, and, where appropriate, by using the thermal capacity of the machine to even out peak or cyclic thermal loads. The superior dissipation obtained by blowing air over a surface rather than relying on natural convection and radiation results in all but the smallest motors relying on some form of forced cooling, in which cooling air is blown over the surfaces to be cooled by shaft-driven fans. Though the cooling-air paths of electric motors are often complicated by the need to cool all the many component parts of the machine in which losses occur and therefore in which heat is generated, nevertheless an application of quite simple considerations of fluid flow yield acceptable methods of estimating the resistance of the air paths. Similarly, the behaviour of the common types of fans used on electrical machines is developed, and, by way of illustration, graphical solutions are indicated for a few typical motor cooling circuits.