  In the perturbative approach to quantum field theory it is common to replace
the propagator $i (p^{2}-m_{0}^{2}+i\varepsilon )^{-1}$ for a scalar field by a
similar expression, namely $iZ (p^{2}-m^{2}+i\varepsilon )^{-1}$, where the
shift of the mass from $m_{0}$ to $m$ reflects the mass renormalization and the
constant~$Z$ is the renormalized field strength (or wave-function). We argue
that, contrary to general belief, the non-perturbative value of~$Z$ is not
necessarily equal to zero in case the two-point function of an interacting
quantum field theory is, as expected, more singular on the light-cone than the
corresponding free field two-point function. If, however, (massless) photons or
composite (unstable) particles are present, the condition $Z=0$ follows from
two qualitatively different arguments, one being a theorem due to Buchholz, the
other a criterion due to Weinberg. Hence, the condition $Z=0$ is, after all, a
universal feature of realistic models of elementary particle physics, which
include massless or unstable particles. The results hold within a natural
framework which, in the case of gauge theories, requires Hilbert space
positivity, and therefore the use of non-manifestly covariant gauges.
