The interaction of hydrogenic atoms with a weak constant magnetic field is discussedindetail.The Breit Hamiltonian, minimally coupled to the external magnetic field, is treated in severaldifferent ways. First, approximate eigenfunctions are obtained in the nonrelativistic nucleusapproximation. These wave functions are used to treat perturbatively the residual terms dependenton the magnetic field, and to identify the magnetic moment of the bound electron in theground state, The corrections previously given by us, of relative order (Ze), (Ze) m/M, e (Zn),and 0.'(Zn) m/M, are confirmed including lowest-order radiative corrections. Next, a unitary transformationof the complete Breit Hamiltonian is made in order to simplify further the calculationof small corrections to the ejectron and nuclear g factors. The physical origin of this unitarytransformation, which is similar to a gauge transformation. , is discussed extensively, and it isshown that the transformed Hamiltonian for a neutral system commutes with P, the momentumconjugate to the center-of-mass position X. This new Hamiltonian, which treats the electronand the nucleus on equal footing, is then transformed by means of the Chraplyvy-Barker-Gloverreduction. The electron and the nuclear g factors are calculated, this time including terms ofrelative order (Zo. ) m /M and e(ZQ. ) m /M2. These computations yield the magnetic momentsfor hydrogenic atoms in their ground