Gauge-Independence and the Two-Body Problem in QED

I describe a gauge-independent approach to the relativistic two-body bound state and scattering problems in quantum field theory. The basic tool is an ordinary three-dimensional equation involving a potential operator V which gets contributions from both irreducible and reducible diagrams. In QED the resultant V is independent of the choice of covariant gauge used for the photon propagator, unlike the kernel in the Bethe–Salpeter equation. As an illustration, a problem concerning spin-independent level shifts in two-body bound states is analyzed.