  We review in this article the role which the work of Pauli and Weisskopf
played in formulating a quantum field theory of spinless particles. To make our
computations as transparent as possible, we offer a physicist's derivation of
the Klein-Gordon-Fock equation. Since invariant functions play a significant
part in our paper, we will discuss them in great detail. We emphasize Pauli's
and Weisskopf's view that Dirac's hole theory is totally obsolete in
formulating a consistent quantum field theory, be it for scalar or spinor
particles. As an important example we present the calculation for producing
charged scalar particles in an external electric field.
