Chapter 6 treated at length the instability of the neutral Dirac vacuum in a sufficiently strong, i.e. supercritical, external field and its spontaneous decay into a charged vacuum. The discussion then was based on Dirac’s intuitive hole picture of the electron — positron vacuum combined with the theory of spontaneous autoionization processes. While such an amalgamation of intuitive, pictorial ideas and relativistic quantum mechanics may serve as a useful guide to the underlying physical phenomena, it is equally clear that it cannot rigorously prove our assertions. This proof should more properly be sought within the framework of relativistic quantum field theory which enables a consistent description of processes involving virtual and real particle creation.