The phenomenon of ionization in the dielectric has been selected for detailed discussion in this paper, as it lies at the root of success or failure in the operation of high-voltage cables. The two leading features of the paper are a fairly complete co-ordination and critical examination of the published work on the subject and an account of some of the author's own investigations made with the object of confirming existing suggestions and extending our knowledge along certain lines to still wider limits. As ionization is usually-demonstrated by its effects on the variation of power factor with voltage, several pages are devoted to different phases of this important relationship, and the physical mechanism controlling the appearance or modification of ionization is then reviewed and illustrated. The close connection between ionization and long-time breakdown strength receives some attention, and tentative suggestions are made for the assessment of quality. A noteworthy feature of the paper are the data demonstrating the advance which has been made in applying a knowledge of the phenomenon of ionization to the production of reliable solid-type high-voltage cable.