Recent experimental work on the insulating properties of high vacuum is systematically reviewed. A résumé of the prebreakdown conduction phenomena is given; in small gaps, the steady prebreakdown currents appear to be due to a modified type of field emission, while in longer gaps the contamination on the electrode surfaces gives rise to an ion-exchange mechanism, which is responsible for the microdischarges associated with these gaps. The parameters affecting the breakdown strength of a vacuum gap, and the different hypotheses postulated to explain the mechanism of breakdown, are considered in detail. No single hypothesis is able to account for all the experimental results reported, but it appears that a transition from one type of mechanism to another takes place as the interelectrode gap is increased.The paper also considers the important practical aspects of breakdown across solid insulating surfaces in vacuum.