In the quantum case the problem is to find a set of ten self-adjoint operators that satisfy the Poincaré Lie algebra. In the absence of interactions, Stone's theorem applied to tensor products of known unitary irreducible representations of the Poincaré group gives a set of self-adjoint light-front generators with all of the required properties. The problem of adding interactions is no different than it is in non-relativistic quantum mechanics, except that the added interactions also need to preserve the commutation relations.