A critical consideration of the theories of dielectric breakdown in ionic crystals proposed by Seeger and Teller and by Fr\"ohlich is given. It is shown that the main difference in the formulae for the breakdown field is due to the fact that Seeger and Teller make the unjustified assumption that an electron transfers energy to the lattice vibration without changing its direction of motion. A mistake in the interaction between an electron and an ion, used by Seeger and Teller, is corrected. The author's condition for breakdown is discussed in greater detail than has been done hitherto.