  Gauge-invariant perturbation theory for theories with a Brout-Englert-Higgs
effect, as developed by Fr\"ohlich, Morchio and Strocchi, starts out from
physical, exactly gauge-invariant quantities as initial and final states. These
are composite operators, and can thus be considered as bound states. In case of
the standard model, this reduces almost entirely to conventional perturbation
theory. This explains the success of conventional perturbation theory for the
standard model. However, this is due to the special structure of the standard
model, and it is not guaranteed to be the case for other theories. Here, we
review gauge-invariant perturbation theory. Especially, we show how it can be
applied and that it is little more complicated than conventional perturbation
theory, and that it is often possible to utilize existing results of
conventional perturbation theory. Finally, we present tests of the predictions
of gauge-invariant perturbation theory, using lattice gauge theory, in three
different settings. In one case, the results coincide with conventional
perturbation theory and with the lattice results. In a second case, it appears
that the results of gauge-invariant perturbation theory agree with the lattice,
but differ from conventional perturbation theory. In the third case both
approaches fail due to quantum fluctuations.
