  In quantum field theory the concept of a Lagrangian interaction density,
expressed in terms of fields, is primary. Forces between two particles are
regarded as arising primarily from the exchange of quanta of the bosonic
fields. Thus, in contrast to nonrelativistic quantum mechanics, the concept of
a two-body poten tial is secondary. Potentials are not given a priori but must
be defined. Part of the purpose of this talk is to review and discuss the
issues involved when such definitions are made. In this context I describe a
gauge-independent approach to some aspects of the problem of determining the
energy levels of bound states, developed in collaboration with the late G.
Feinberg, and report some recent results on the long-range potential associated
with two-photon exchange between charged particles.
