  Effective Lagrangians were originally used only at the tree level as
so-called phenomenological Lagrangians since they were in general
non-renormalizable. Today they are treated as effective field theories valid
below a characteristic energy scale. Quantum corrections can then be calculated
in a consistent way as for any renormalizable theory. A few applications of the
Euler-Heisenberg Lagrangian for interacting photons at low energies are
presented together with recent developments in the use of QED for
non-relativistic systems. Finally, the ingredients of an effective theory for
the electroweak sector of the Standard Model are discussed in the case of a
non-linear realization of the Higgs mechanism using the Stuckelberg formalism.
[Invited talk at "Beyond the Standard Model V", Balholm, Norway, April 29 - May
4, 1997]
