Introduction to recoil effects in bound state problems

A description is given of some of the considerations which are necessary to treat two-body hydrogen-like systems when the electron's relativistic properties must be taken into account. Most modern approaches to the two-body problem are based on the physics of the Bethe-Salpeter approach, although the methodology is different. One uses some three-dimensional reference equation which incorporates as much physics as possible and the corrections appears as four-dimensinal kernels which are treated by a procedure resembling usual perturbuation theory. An example is given where both reduced mass effects and relativistic effects are treated in a unified approach