  The description of electromagnetic reactions at intermediate energies, such
as pion electroproduction or (virtual) Compton scattering, traditionally starts
from covariant tree-level Feynman diagrams (Born or pole terms). Internal
hadron structure is included by means of (on-shell) form factors in the
vertices while free propagators are used. To overcome problems with gauge
invariance, simple prescriptions such as choosing $F_1^V (q^2) = F_{\pi} (q^2)$
in pion electroproduction or the ``minimal substitution'' are used. We discuss
the limitations of such approaches by comparing to the most general structure
of electromagnetic vertices and propagators for pions and nucleons. The recipes
to enforce gauge invariance are critically examined and contrasted with the
exact treatment. The interplay between off-shell effects and irreducible
``contact'' terms is demonstrated for real Compton scattering on a pion. The
need for a consistent microscopic treatment of reaction amplitudes is stressed.
Shortcomings of minimal substitution are illustrated through an example in the
framework of chiral perturbation theory.
