After the relative importance of the phenomenon of tracking has been mentioned and defined, the methods available for measuring the susceptibility of insulating materials to tracking are described. A simple test method is given which, it is suggested, could be adopted as standard test. By the use of this method, an examination has been made of some of the variables involved in testing for tracking susceptibility, in order to improve and assess the reliability of results.Attention is drawn to some further information which is required and which, it is hoped, will be soon forthcoming. In particular, the intra- and inter-laboratory reproducibility of the test method must be examined.