Recent investigations are reviewed of prebreakdown conduction phenomena in vacuum both for steady and for step-function applied voltages. The results of these investigations have led to the formulation of models to explain the initiation of the vacuum discharge by ionization of a vapor medium, the vapor being composed of atoms of one or both of the electrodes. A detailed examination of the events leading to breakdown has allowed a more critical evaluation of the merits and deficiencies of the various breakdown models.