  After a general introduction to the structure of effective field theories,
the main ingredients of chiral perturbation theory are reviewed. Applications
include the light quark mass ratios and pion-pion scattering to two-loop
accuracy. In the pion-nucleon system, the linear sigma model is contrasted with
chiral perturbation theory. The heavy-nucleon expansion is used to construct
the effective pion-nucleon Lagrangian to third order in the low-energy
expansion, with applications to nucleon Compton scattering.
