The nature of recent international research collaboration is described. Breakdown data obtained on a wide variety of air clearances are comprehensively summarised; the effects of the polarity and shape of the testing impulse, the geometry of the electrodes and the condition of the atmosphere are considered separately. Recent improvements in the physical understanding of the pre-breakdown phenomena are discussed in some detail, and their application to engineering problems, by means of models, is considered. Newly developed methods of field calculation are applied to complex configurations subjected to more than one voltage stress. Statistical methods are used, both in assigning confidence intervals to test data, and in predicting risks of failure.