  It is often stated that the vacuum is trivial when light-front (null-plane)
quantization is applied to a quantum field theory, in contrast to the situation
with equal-time quantization. In fact, it is has long been known that the
statement is false, and that in certain cases the standard rules for
light-front perturbation theory need modification. This paper gives an
elementary review of these issues, including an explanation of how and when
there is a failure of the elementary derivation of the rules for light-front
perturbation theory.
