  This dissertation presents the first theoretical investigation of the Lamb
shift in a light-front hamiltonian approach: the dominant part of the splitting
between the 2S(1/2) and 2P(1/2) energy levels in hydrogen is calculated. Also
presented for the first time is an analytic calculation in a light-front
hamiltonian approach of the singlet-triplet spin splitting in the ground state
of positronium through fourth order in the fine-structure constant.
