  We construct Hamiltonians for systems of nonrelativistic particles linearly
coupled to massive scalar bosons using abstract boundary conditions. The
construction yields an explicit characterisation of the domain of
self-adjointness in terms of boundary conditions that relate sectors with
different numbers of bosons. We treat both models in which the Hamiltonian may
be defined as a form perturbation of the free operator, such as Fr\"ohlich's
polaron, and renormalisable models, such as the massive Nelson model.
