  After a brief survey of effective field theories, the linear sigma model is
discussed as a prototype of an effective field theory of the standard model
below the chiral-symmetry-breaking scale. Although it can serve as a toy model
for the pion-nucleon interaction, the linear sigma model is not a realistic
alternative to chiral perturbation theory. The heavy-baryon approach to chiral
perturbation theory allows for a systematic low-energy expansion of Green
functions and amplitudes with baryons. The chiral-invariant renormalization of
the effective field theory for the pion-nucleon system to O(p^3) in the chiral
expansion is reviewed.
