  We consider the nonrelativistic quantum mechanics of a model of two spinless
fermions interacting via a two-body potential. We introduce quantum fields
associated with the two particles as well as the expansion of these fields in
asymptotic ``in'' and ``out'' fields, including such fields for bound states,
in principle. We limit our explicit discussion to a two-body bound state. In
this context we discuss the implications of the Galilean invariance of the
model and, in particular, show how to include bound states in a strictly
Galilean-invariant quantum field theory.
