  Starting from a theory of heavy particles and antiparticles, the path
integral formulation of an effective field theory which describes the low
momentum interactions is presented. The heavy degrees of freedom are identified
and explicitly integrated out from the functional integral of the full theory.
Using this method, the effective Lagrangian, which may be calculated to
arbitrary subleading order in an inverse mass expansion, is derived for fields
of spin-0, 1/2, and 1.
