atominto conventional c.m. and relative variables guaranteesthat the total atomic wave function is knownonce the c.m. wave function is known. RelativisticHamiltonians such as the Breit Hamiltonian are notseparable in the above sense and therefore theproblem of determining the complete wave functionis nontrivial. This nonseparability is the basicproblem which must be surmounted in order tohandle the problem of electromagnetic interactions.Recently a number of papers have considered theelectromagnetic interactions of loosely bound systemsby various methods. ' ' The present papersuggests another approach but considers in detailonly the interaction of hydrogenic atoms in a magneticfield. It contains a more comprehensivetreatment of work previously discussed morebriefly by both authors.In Sec. II we show that approximate solutions ofthe Breit equation may be obtained to describe themotion of the atom. We discuss the approximatenature of these solutions. In Sec. III we treat theimportant problem of the hydrogenic atom in anexternal magnetic field. Only the ground state isdiscussed here although in future work it should beeasy to analyze excited states. We calculate boundstatecorrections to the electron g factor includingterms of order (Zn)2m/M Sectio. n III, as well asSec. IV, in which the radiative corrections areanalyzed, gives a detailed treatment of earlierwork published by one of us (H. G. ). ' Since thewave functions are not accurate enough to calculatethe corrections of order (Zn)ama/Ma, further accuracyis achieved by using the Barker-Glovermethod, the approach previously discussed by oneof us (H. A. H. ).' In Sec. V we discuss a, unitarytransformation of the Hamiltonian which greatlysimplifies the calculations. This Hamiltonian alsoincludes the anomalous moment terms discussedearlier. In Sec. VI the Barker-Glover method isused to transform the Hamiltonian of Sec. V to aform in which the operators a.re even (with respectto Dirac matrices) in both electron and nuclearvariables. Both the electron and the proton g factorsare obtained for the 1S state of hydrogen.Finally, in Sec. VII we summ, arize the results andmake some comparisons with experiment.