Perturbative methods allow accurate calculations of QED bound states (atoms). Hadrons have atom-like features, even though their quark and gluon constituents are highly relativistic and confined. The possibility that rigorous Hamiltonian methods can be useful for QCD bound states merits careful attention. par Familiar concepts such as “Born term” and “inclusive probability” are relevant also for bound states. They allow to formulate a zeroth order approximation for mesons and baryons which can replace the free “in” and “out” states in a perturbative $alpha_s$ expansion. A linear $A^0$ potential follows from Gauss’ law when one imposes a non-vanishing, universal field energy at spatial infinity. The requirements of Poincaré and gauge invariance are satisfied in a non-trivial way. Previous lecture notes can be found at http://arxiv.org/abs/1402.5005 and a summary of results at http://arxiv.org/abs/1409.4703 .

University of Helsinki